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PB 1990-2009




                                                                                                                                                                     DWS
               HIGH PERFORMANCE BOARD
                        DESIGN

FREQUENCY                                                            LIMIT                                                                                                                                                                             LOGIC/TIMING CORE



TEST                                                                                                                                                                                                                                                                                             OUT SHAPING NET

                                                                                                                                                                                                                                                                                                 L
                                                                                                                                                                                                                                                                                                               no T   PWLO
                                                                                                                                                                                                                                                 IN                                  OUT
                                                                                                                                                                                                                                                              nT TH
It is well known that the digital                                                                                                                                                                                                  +
                                                                                                                                                                                                                                                                                                     C
                                                                                                                                                                                                                                                                                                                               +          LO
                                                                                                                                                  TPKI                                                                                                                                                                                           TPKO
bandwidth in terms of maximum                                                                                                                                                                       ni T        PWLI
                                                                                                                                    IN                               R                                                                                                                                                             P RO                   OUT
allowable clock frequency is a key                                                                                                                   PRI                                     50 K                                                                             I
                                                                                                                                                                                                                                                                                                                                          RO

parameter to be evaluated during                                                                                                                                                            VRI
                                                                                                                                                                     C
both the design and validation
cycles      of    high-performance                                                                                                                                                                                                               VC
                                                                                                                                                                                                        V                                                                    PWLO
systems.                                                                                                                                                                        PWLI                     C
                                                                                                                                                                                                                                            0                     1
System "soft" failures due to                                                                                                                                                                       1
                                                                                                                                                                                                                                                                               VI

electrical and timing problems are                                                                                                                                                                                                      -.8                                                              Total delay = T PKI + T PKO+ ni T + no T + n T

identified by testing the system at                                                                                                                                  -1.7 -.9
                                                                                                                                                                                                    0
                                                                                                                                                                                                              VI
operating speeds exceeding those                                                                                                                                         V
                                                                                                                                                                          BB
required for nominal operation.
The margin in terms of difference                                                                                      Fig. 1: Scheme of the electrical macromodel for input-output ECL ports.
between nominal and maximum
                                                                                                                                                                                                                                                                                                                      frequency is a good
                                                                                                                                                                                                                                                                                                                      figure for evaluating
                                                                                                                                                                                                                                                                                      DATA OUT
                                                                                                                                                                                                                                                                                                                      system robustness and
                                                                                                                          RPD13
                                                                                                                           430            40             2
                                                                                                                                                                    U5A
                                                                                                                                                                                4                                          TXOR2
                                                                                                                                                                                                                                                                                                                      reliability.
                                                                                                         Vee

                                                                                                                                           30            3                      5
                                                                                                                                                                                                                           75 ohm
                                                                                                                                                                                                                            300 ps                                                                                    This application note
                                                                                                                          RPD14                                     100107

                                                                                                           TDAT
                                                                                                                           430
                                                                                                                                         RS3                                                                                                                                                                          describes the way in
                                                                      Vee
                                                                                                           75 ohm
                                                                                                           300 ps
                                                                                                                          Vee
                                                                                                                                         68                                         TXOR1

                                                                                                                                                                                    75 ohm
                                                                                                                                                                                    300 ps
                                                                                                                                                                                                                                                        Vee
                                                                                                                                                                                                                                                                                    RS2
                                                                                                                                                                                                                                                                                                                      which DWS can be
                                                                            RPD3
                                                                            430
                                                                                                                                RPD5
                                                                                                                                430
                                                                                                                                                                          RS1
                                                                                                                                                                          68

                                                                                                                                                                                        TINT3                                                TINT4
                                                                                                                                                                                                                                                              RPD10
                                                                                                                                                                                                                                                              430
                                                                                                                                                                                                                                                                                    68

                                                                                                                                                                                                                                                                                      RPD11
                                                                                                                                                                                                                                                                                       430
                                                                                                                                                                                                                                                                                                                      effectively used for
                          6                7                TINT1                          11              TINT2                                     7                                                  24                 11                                                 11


                          5
                              U2C
                              100131FP
                                               8
                                                          75 ohm
                                                          100 ps
                                                                            1
                                                                              U2B
                                                                           24 100131FP
                                                                                             12
                                                                                                           75 ohm
                                                                                                           200 ps
                                                                                                                                5
                                                                                                                                  U3C
                                                                                                                                6 100131FP
                                                                                                                                                     8
                                                                                                                                                                                       75 ohm
                                                                                                                                                                                       100 ps
                                                                                                                                                                                                          1
                                                                                                                                                                                                              U3B
                                                                                                                                                                                                              100131FP
                                                                                                                                                                                                                            12
                                                                                                                                                                                                                                            75 ohm
                                                                                                                                                                                                                                            200 ps
                                                                                                                                                                                                                                                             U4B
                                                                                                                                                                                                                                                          24 100131FP
                                                                                                                                                                                                                                                                               12
                                                                                                                                                                                                                                                                                                         Vee          optimizing the design of
                                                                                                                                                                                                                                                              1

                                   4   3           RPD4
                                                   330
                                                               RS4
                                                               68
                                                                                     2
                                                                                                   RPD6
                                                                                                   330
                                                                                                                                     4        3              RPD8
                                                                                                                                                             430
                                                                                                                                                                       RPD7
                                                                                                                                                                       430
                                                                                                                                                                                                                                 RPD9
                                                                                                                                                                                                                                 330                                  2
                                                                                                                                                                                                                                                                                      RPD12
                                                                                                                                                                                                                                                                                       330                            a high speed ECL 100K
                                           Vee                                               Vee
                                                                                                                                                                          Vee                                                                                                                                         board,             taking
                                                                                                    TD

                                                                                                  75 ohm
                                                                                                  300 ps
                                                                                                                                                                                                                                                                                                                      simultaneously       into
                                                            TCK1
                                                                                                                             TCK2
                                                                                                                             75 ohm
                                                                                                                                                                                                                17
                                                                                                                                                                                                                     U6A
                                                                                                                                                                                                                                            15
                                                                                                                                                                                                                                                                               TD1
                                                                                                                                                                                                                                                                          75 ohm 100 ps
                                                                                                                                                                                                                                                                                                 DATA OUT
                                                                                                                                                                                                                                                                                                                      account the problems of
                                                                                                                             200 ps
                                                            75 ohm
                                                            200 ps
                                                                                                       RSCK1
                                                                                                          47
                                                                                                                                                                                                  RPD21
                                                                                                                                                                                                                18

                                                                                                                                                                                                                     100102FP          RPD15
                                                                                                                                                                                                                                            16

                                                                                                                                                                                                                                                              RPD16
                                                                                                                                                                                                                                                                          75 ohm 100 ps
                                                                                                                                                                                                                                                                                TD2
                                                                                                                                                                                                                                                                                                 DATA OUT*
                                                                                                                                                                                                                                                                                                                      signal degradation due
                                                                                     U1A                                                                                                           10K                                   330                  330
                                                                                                                                RSCK2
                                                                                                                                                                                                                                                                                                                      to        interconnection
                  U7A                                                                                15
                                                                                17
               100114FP                                                                                                         47    RSCK3
                                                    TC1                                                                               47                             TCK3                               Vee
                    17        16                                                                                                                                                                                                                 Vee    Vee
  CK IN

  CK IN*
                    18         15
                                                   75 ohm
                                                   100ps                   RPD9
                                                                           330
                                                                               18
                                                                                  100102FP
                                                                                                     16               RPD2
                                                                                                                      330
                                                                                                                                               Vee
                                                                                                                                                                    75 ohm
                                                                                                                                                                    200 ps
                                                                                                                                                                                                                                                                                                                      effects and logic-timing
              RT1
              100
                                                                     Vee
                                                                                                                      RPD1

                                                                                                                      330
                                                                                                                                                                                                                                                                                                                      problems related to
                                                                                                                                                                                                                     U1D                                                      TCK01
                                                                                                                    TC2

                                                                                                                    75 ohm
                                                                                                                    100 ps
                                                                                                                                                                                                                 1                      7                                 75 ohm 100 ps
                                                                                                                                                                                                                                                                                                 CKOUT                actual behaviour of
                                                                                                                                                                                                                                                                                                 CKOUT*

                                                                                                                                                                                    RPD19
                                                                                                                                                                                     330
                                                                                                                                                                                                  RPD20
                                                                                                                                                                                                    10K
                                                                                                                                                                                                                 2
                                                                                                                                                                                                                     100102FP
                                                                                                                                                                                                                                        8
                                                                                                                                                                                                                                        RPD17
                                                                                                                                                                                                                                          330
                                                                                                                                                                                                                                                                  RPD18
                                                                                                                                                                                                                                                                  330
                                                                                                                                                                                                                                                                          75 ohm 100 ps
                                                                                                                                                                                                                                                                              TCK02                                   active devices.
                                                                                                                                                                                            Vee         Vee                                      Vee     Vee




Fig.2 : Initial pre-layout version of the 31-bit pseudo-random generator board (PN5DB).

Copyright Piero Belforte
HIGH SPEED                          BOARD                                                                                                                                                 -2 V


MODEL                                                                                                                                                                                            RT ERM
                                                                                                                             PN50PT
                                                                                                                                                                                                   75
                                                                                                                      300                                                           100
100K logic gates and flip-flops are                                                                                   400
                                                                                                                              Data (U2A)

                                                                                                                                CK (U2A)
                                                                                                                                                                 DAT A_OUT
                                                                                                                                                                                    200
                                                                                                                                                                 DAT A_OUT 
accurately modelled at electrical-
                                                                                                                      500
timing-logic levels, including such                                                                                             CK_IN
                                                                                                                      600
key issues as timing skew between                                                                                                  CK_IN

                                                                              T cable1                     T cable2                         reset                  set
outputs, internal path delay                                                  50 ohm                       50 ohm
                                                                              2 ns                         2 ns
mismatches      and     input-output                                          ideal cable                  ideal cable

electrical equivalents obtained by                                      501                                601


taking into account package and                                               R
                                                                              50
                                                                                                           R
                                                                                                           50

nonlinear effects as observed in                                 VCP1                                        VCP2
device characterization (Fig. 1). In
                                                                                                                                                                         V3
this particular case, an equivalent                                                            VCH
                                                                                               1.3 volt
                                                                                                                                                    V2
                                                                                                                                                    V reset              V set
                                                                                                                                                    2 ns pulse           1.7 volt
electrical model for input-output                                SWGEN


ports is used instead of the more
direct s-parameter behavioural                               Fig. 3: Electrical scheme of the test set-up used to evaluate the limit frequency of
                                                             the PRBS board.
model used for other applications.

-0.50 V
                                                                                                                                                    Despite the complexity of the
                                                                                                                                                    resulting model network (on the
-1.30 V
                                                                                                                             V(100)
                                                                                                                             layout4.g
                                                                                                                                                    order of a thousand elements) and
-2.00 V
-0.50 V
                                                                                                    * 245 MHz
                                                                                                                                                    the number of calculated time-
                                                                                                                                                    points necessary to achieve a good
-1.30 V
                                                                                                                             V(100)
                                                                                                                             layout3.g
                                                                                                                                                    resolution (on the order of several
-2.00 V
-0.50 V
                                                                              * 239 MHz
                                                                                                                                                    thousand),      DWS runs require
                                                                                                                                                    few milliseconds on a standard PC
-1.30 V
                                                                                                                             V(100)
                                                                                                                             layout2.g              ,   so     that   the    interactive
                                                                 * 235 MHz
-2.00 V
-0.50 V                                                                                                                                             optimization       recycles      are
-1.30 V
                                                                                                                                                    accomplished in a small amount of
                                                                                                                             V(100)

                                                                                      * frequency limit
                                                                                                                             layout1.g              time.
-2.00 V                                   * 218 MHz
-0.50 V


-1.30 V
                                                                                                                             V(400)
                                                                                                                                                    DESIGN OPTIMIZATION
                                                                                                                             layout1.g
-2.00 V

      0.50       0.55    0.60    0.65    0.70        0.75       0.80          0.85          0.90          0.95           1.00                       Starting with an initial pre-layout
                                                                                                                            TIME[uS]                version of the pseudo-random
Fig. 4a: Layout impact on performance: swept freq. test (200-250 MHz)                                                                               generator        board      (PN5DB)
 -0.50 V
                                                                                                                                                    including interconnects impedance
                                                                                                                                                    and length evaluation (Fig. 2),
 -1.30 V
                                                                                                                              V(200)
                                                                                                                              layout4.g
                                                                                                                                                    DWS simulation easily pinpoint
 -2.00 V
 -0.50 V
                                                                                                                                                    the causes of failures when
                                                                                                           * 245 MHz
                                                                                                                                                    working       at     higher    clock
 -1.30 V
                                                                                                                              V(200)
                                                                                                                              layout3.g
                                                                                                                                                    frequencies. To achieve this goal a
 -2.00 V
 -0.50 V
                                                                                                                                                    limit frequency test setup is
                                                                                              * 239 MHz
                                                                                                                                                    simulated as shown in Fig.3.
 -1.30 V
                                                                                                                              V(200)
                                                                                                                              layout2.g
                                                                                                                                                    The board is connected to a sweep
 -2.00 V
 -0.50 V                                                               * 235 MHz
                                                                                                                                                    frequency clock generator and the
                                                                                                                                                    most significant board signals are
 -1.30 V
                                                                                                                              V(200)
                                                                                                                              layout1.g             monitored in order to observe
 -2.00 V
 -0.50 V                                 * 218 MHz
                                                                                                                                                    when a failure occurs.
 -1.30 V
                                                                                                                                                    Typical failures consist of an
                                                                                                                              V(400)
                                                                                                                              layout1.g             incorrect bit sequence with respect
 -2.00 V
                                                                                                                                                    to a pseudo-random 31-bit
          0.50    0.55    0.60    0.65    0.70        0.75       0.80          0.85          0.90          0.95             1.00

                                                                                                                            TIME[uS]

Fig. 4b: Layout impact on performance: swept freq. test (200-250 MHz)
                                                                                                                                                                                                          2
Copyright Piero Belforte
sequence expected during correct                                                                     overshoots                                           due                       to               reflections                                 violation on the first 100131 D-
                                                                                                                                                                                                                                                 type master-slave flip-flop (U2A)
                                                                                                                                                                                                                                                 causing detestable operation.
                                                                                                  LAYOUT VERSION                                                                                                                                 After this first result several design
                                                                                                                                                                                                                                                 modifications have been evaluated
                                                                               #1                                      #2                                 #3                                 # 4 (opt)
                                                                                                                                                                                                                                                 in order to identify the best
                                        TCK1                              300                                    200                                     200                                         200                                         solution and the impact on
          TRACE
                                                                                                                                                                                                                                                 performance is summarized in Fig.
    ELECTRICAL                          TCK2                              100                                    200                                      200                                        200                                         5. The most relevant improvement
                                                                                                                                                                                                                                                 in terms of clock speed is obtained
          LENGTH
                                        TCK3                              100                                    200                                     200                                         200                                         by shortening the delay of input-
           ( ps )                                                                                                                                                                                                                                output traces of the exclusive-or
                                        TDAT                              300                                     300                                     200                                        100                                         gate U5A (100107) from 300ps (~
                                                                                                                                                                                                                                                 6cm) to 100ps (~ 2cm) and trying
          F max.                        sweep                             218                                     235                                     239                                        245                                         to increase the delay of the clock
                                                                                                                                                                                                                                                 path of U2A with respect the
          ( MHz )                                                                                                                                                                                                                                other flip-flops in the chain. This
                                        fixed                                    -                                      -                                            -                             238.1
                                                                                                                                                                                                                                                 last adjustment is implemented
Fig. 5: Summary of the limit frequency versus layout parameters: trace length has great                                                                                                                                                          utilizing the clock driver gate
impact on performance                                                                                                                                                                                                                            U1A (100102) as delay for U2A
                                                                                                                                                                                                                                                 and U2B clock inputs and
operation (Fig. 4).                                                                                  within device package, output trace                                                                                                         connecting the other flip-flops

                                                                                                                                                                                                                            -2 V




                                                                                                                                                                                                                  RPD7              RPD11
                                                                                                                                                               RPD13                                               75               75
                                                                                                                                                                                                     U5D                                               TXOR2
                                                                                                                                                                430                      7                    1
                                                                                                                                               Vee
                                                                                                                                                                                                                                                           75 ohm
                                                                                                           TDAT
                                                                                                                                                                                                                                                            100 ps

                                                                                                           75 ohm                                                                        6                    24
                                                                                                                                                                                                   100107FP
                                                                                                           100 ps




                                                                                                                                                                                                                   TXOR1


                                                                                                    Vee                                                        Vee                                                 75 ohm                                                                 Vee
                                                                                                                                                                                                                   300 ps


                                               300                                                        RPD3                                                       RPD5                                                                                                                       RPD10
                                                           U2A
                                                                                                          430                                                        430                                                                                                                        430
                                         RPD14             100131FP                                                                                                                                                    TINT3                                                   TINT4
                                                      15                  14             TINT1                                11                 TINT2                               7                                                24                   11                                                    11
                                 -2 V                                                                           U2B                                                      U3C                                                                U3B                                                   U4B
                                                      16                                                  24    100131FP                        75 ohm            6      100131FP                                                           100131FP                       75 ohm              24 100131FP
                                          75                                            75 ohm                                                                                                                        75 ohm
                                                                                        100 ps                                                  200 ps                                                                100 ps                                               200 ps                                       RPD12
                                                      20                  13                                                   12                                                    8                                                                      12                                                   12
                                                                                                          1                                                       5                                                                    1
                                           400                                                                                                                                                                                                                                                  1                                        Vee
                                                                                                                                                                                                                                                                 RPD9                                                     330
                                                                                 RPD4       RS4                                         RPD6                             4    3              RPD8
                                                               4      3                                           2                                                                                                                          2                                                            2
                                                                                 330                                                    330                                                  430                                                                 330
                                                                                            68

                                                                                                                                                                                             Vee
                                                                           Vee                                                 Vee
                                                                                                                                        TCK2

                                                                                                                                    75 ohm
                                                                                                                                    200 ps
                                                                                                                                                                                  TCK3

                                                                                                                                                                              75 ohm
                                                                                                                                                                              200 ps                                                                                                                                        TD1
                                                                                                                                                                                                                                                                     U6A
                                                                                                                                                                                     TD                                                                         17                        15                           75 ohm 100 ps     100
                                                                                                                                                                                                                                                                                                                                                   DATA OUT
                                                                                                                                         TCK1                                       75 ohm
                                                                                                                                                                                    300 ps                                                                      18                        16
                                                                                                                                                                                                                                                                                                                                                   DATA OUT
                                                                                                                                        75 ohm                                                                                                                                                                         75 ohm   100 ps   200
                                                                                                                                        200 ps                                                                                               RPD21                   100102FP            RPD15
                                                                                                                      Vee                                                                                                                                                                                                     TD2
                                                                                                                                                                                                                                              10K                                          330

                                                                                                               RPD22                                       RSCK1
                                                                                                                10K                                           47                                                                                     Vee                                            Vee
                                                                                                                                        U1A
                                                  U7D                                                                                                    15
                                                                                                                                   17
                     TCIN1                     100114FP
           500   50 ohm 200 ps                                                                   TC1
                                                1          8
  CK IN
                                                                                                 75 ohm                            18                     16
                                                                                                                                        100102FP
 CK IN                                                                                          100ps              RPD9                                                     330
           600                                  2          7                                                        330                                                                            Vee
                 50 ohm 200 ps
                     TCIN2               RT1                                                                                                                                 RPD1
                                         100                                                                                -2 V
                                                                                                                                                                             330




                                                                                                                                                                                                                                                                                                                  TCK01
                                                                                                                                                                                                                                                     U1D
                                                                                                                                                          TC2                                                                                    1                         7                                  75 ohm 100 ps
                                                                                                                                                                                                                                                                                                                                          CKOUT
                                                                                                                                                         75 ohm
                                                                                                                                                         100 ps
                                                                                                                                                                                                                                                                                                                                          CKOUT
                                                                                                                                                                                                                                                 2                         8
                                                                                                                                                                                                                                                                                                              75 ohm 100 ps
                                                                                                                                                                                                                   RPD19           RPD20             100102FP              RPD17                     RPD18        TCK02
                                                                                                                                                                                                                    330              10K                                     330                     330


                                                                                                                                                                                                                            Vee        Vee                                         Vee         Vee




Fig. 6: Optimized version of the 31-bit pseudo-random generator board (PN50PT)

The      behaviour      differences                                                                  and connector, while output 100,                                                                                                            directly to the output of the
between output       data signals                                                                    being     left  open    for    bus                                                                                                          differential line receiver U7A
V(200) and         V(100)        are                                                                 applications,    has   a     more                                                                                                           (100114) (Fig.6). Minor speed
caused by                                                                                            straightforward behaviour because                                                                                                           improvements are also obtained by
different termination schemes:                                                                       it is terminated off the board                                                                                                              modifying the type of terminations
complementary output 200 has a                                                                       (RTERM visible on Fig. 3).                                                                                                                  used on critical paths turning series
pull down load within the board                                                                      The limit frequency for the initial                                                                                                         into parallel arrangements.
and no termination resistor, so that                                                                 design turns out to be about
the waveform is affected by strong                                                                   200 MHz due to setup time
                                                                                                                                                                                                                                                                                                                                                               3
Copyright Piero Belforte
RESULTS

The final result is the PN5OPT
pre-layout version which operates
correctly up to about 238 MHz in
the fixed frequency test and up to
245 MHz in the swept frequency
set-up showing a 20% performance
improvement over the initial
design.
This result is obtained with board
implementations utilizing active
devices packaged in flat-packs. It
is interesting to evaluate        the
impact on performance due to            -0.50 V
                                                                                                                      * FP
packaging effects:      substituting
larger 24-pin devices for the
                                        -1.30 V
original flat-packs, while leaving                                                                                             V(100)
                                                                                                                               layout4.g

pcb trace lengths unchanged,                                                                                     * 245 MHz
causes a decrease in maximum            -2.00 V
                                        -0.50 V

operating frequency (sweep test)                                                                                 * DIL


from 245 MHz to 229 MHz
showing the great importance of         -1.30 V
                                                                                                                               V(100)
                                                                                                                               pn5db8dil.g
packaging issues for this kind of                                                                * 229 MHz
design (Fig. 7). An analysis            -2.00 V
                                        -0.50 V
procedure      similar     to    that
previously shown can be applied
after the board layout definition in    -1.30 V
                                                                                                                               V(400)
                                                                                                                               pn5db8dil.g
order to verify the way in which
the      differences      in      the   -2.00 V

interconnection paths (length,                0.50   0.55   0.60   0.65   0.70   0.75   0.80   0.85     0.90   0.95          1.00

impedance, via holes, trace                                                                                                   TIME[uS]


proximity, etc.) can affect the         Fig. 7: Maximum operating frequency (sweep test) comparison between flat and 24 pin
                                        dual-in-line package.
performance with respect to the
pre-layout specification.




                                                                                                                                             4
Copyright Piero Belforte

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Mixed mode sim_dws

  • 1. PB 1990-2009 DWS HIGH PERFORMANCE BOARD DESIGN FREQUENCY LIMIT LOGIC/TIMING CORE TEST OUT SHAPING NET L no T PWLO IN OUT nT TH It is well known that the digital + C + LO TPKI TPKO bandwidth in terms of maximum ni T PWLI IN R P RO OUT allowable clock frequency is a key PRI 50 K I RO parameter to be evaluated during VRI C both the design and validation cycles of high-performance VC V PWLO systems. PWLI C 0 1 System "soft" failures due to 1 VI electrical and timing problems are -.8 Total delay = T PKI + T PKO+ ni T + no T + n T identified by testing the system at -1.7 -.9 0 VI operating speeds exceeding those V BB required for nominal operation. The margin in terms of difference Fig. 1: Scheme of the electrical macromodel for input-output ECL ports. between nominal and maximum frequency is a good figure for evaluating DATA OUT system robustness and RPD13 430 40 2 U5A 4 TXOR2 reliability. Vee 30 3 5 75 ohm 300 ps This application note RPD14 100107 TDAT 430 RS3 describes the way in Vee 75 ohm 300 ps Vee 68 TXOR1 75 ohm 300 ps Vee RS2 which DWS can be RPD3 430 RPD5 430 RS1 68 TINT3 TINT4 RPD10 430 68 RPD11 430 effectively used for 6 7 TINT1 11 TINT2 7 24 11 11 5 U2C 100131FP 8 75 ohm 100 ps 1 U2B 24 100131FP 12 75 ohm 200 ps 5 U3C 6 100131FP 8 75 ohm 100 ps 1 U3B 100131FP 12 75 ohm 200 ps U4B 24 100131FP 12 Vee optimizing the design of 1 4 3 RPD4 330 RS4 68 2 RPD6 330 4 3 RPD8 430 RPD7 430 RPD9 330 2 RPD12 330 a high speed ECL 100K Vee Vee Vee board, taking TD 75 ohm 300 ps simultaneously into TCK1 TCK2 75 ohm 17 U6A 15 TD1 75 ohm 100 ps DATA OUT account the problems of 200 ps 75 ohm 200 ps RSCK1 47 RPD21 18 100102FP RPD15 16 RPD16 75 ohm 100 ps TD2 DATA OUT* signal degradation due U1A 10K 330 330 RSCK2 to interconnection U7A 15 17 100114FP 47 RSCK3 TC1 47 TCK3 Vee 17 16 Vee Vee CK IN CK IN* 18 15 75 ohm 100ps RPD9 330 18 100102FP 16 RPD2 330 Vee 75 ohm 200 ps effects and logic-timing RT1 100 Vee RPD1 330 problems related to U1D TCK01 TC2 75 ohm 100 ps 1 7 75 ohm 100 ps CKOUT actual behaviour of CKOUT* RPD19 330 RPD20 10K 2 100102FP 8 RPD17 330 RPD18 330 75 ohm 100 ps TCK02 active devices. Vee Vee Vee Vee Fig.2 : Initial pre-layout version of the 31-bit pseudo-random generator board (PN5DB). Copyright Piero Belforte
  • 2. HIGH SPEED BOARD -2 V MODEL RT ERM PN50PT 75 300 100 100K logic gates and flip-flops are 400 Data (U2A) CK (U2A) DAT A_OUT 200 DAT A_OUT accurately modelled at electrical- 500 timing-logic levels, including such CK_IN 600 key issues as timing skew between CK_IN T cable1 T cable2 reset set outputs, internal path delay 50 ohm 50 ohm 2 ns 2 ns mismatches and input-output ideal cable ideal cable electrical equivalents obtained by 501 601 taking into account package and R 50 R 50 nonlinear effects as observed in VCP1 VCP2 device characterization (Fig. 1). In V3 this particular case, an equivalent VCH 1.3 volt V2 V reset V set 2 ns pulse 1.7 volt electrical model for input-output SWGEN ports is used instead of the more direct s-parameter behavioural Fig. 3: Electrical scheme of the test set-up used to evaluate the limit frequency of the PRBS board. model used for other applications. -0.50 V Despite the complexity of the resulting model network (on the -1.30 V V(100) layout4.g order of a thousand elements) and -2.00 V -0.50 V * 245 MHz the number of calculated time- points necessary to achieve a good -1.30 V V(100) layout3.g resolution (on the order of several -2.00 V -0.50 V * 239 MHz thousand), DWS runs require few milliseconds on a standard PC -1.30 V V(100) layout2.g , so that the interactive * 235 MHz -2.00 V -0.50 V optimization recycles are -1.30 V accomplished in a small amount of V(100) * frequency limit layout1.g time. -2.00 V * 218 MHz -0.50 V -1.30 V V(400) DESIGN OPTIMIZATION layout1.g -2.00 V 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Starting with an initial pre-layout TIME[uS] version of the pseudo-random Fig. 4a: Layout impact on performance: swept freq. test (200-250 MHz) generator board (PN5DB) -0.50 V including interconnects impedance and length evaluation (Fig. 2), -1.30 V V(200) layout4.g DWS simulation easily pinpoint -2.00 V -0.50 V the causes of failures when * 245 MHz working at higher clock -1.30 V V(200) layout3.g frequencies. To achieve this goal a -2.00 V -0.50 V limit frequency test setup is * 239 MHz simulated as shown in Fig.3. -1.30 V V(200) layout2.g The board is connected to a sweep -2.00 V -0.50 V * 235 MHz frequency clock generator and the most significant board signals are -1.30 V V(200) layout1.g monitored in order to observe -2.00 V -0.50 V * 218 MHz when a failure occurs. -1.30 V Typical failures consist of an V(400) layout1.g incorrect bit sequence with respect -2.00 V to a pseudo-random 31-bit 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 TIME[uS] Fig. 4b: Layout impact on performance: swept freq. test (200-250 MHz) 2 Copyright Piero Belforte
  • 3. sequence expected during correct overshoots due to reflections violation on the first 100131 D- type master-slave flip-flop (U2A) causing detestable operation. LAYOUT VERSION After this first result several design modifications have been evaluated #1 #2 #3 # 4 (opt) in order to identify the best TCK1 300 200 200 200 solution and the impact on TRACE performance is summarized in Fig. ELECTRICAL TCK2 100 200 200 200 5. The most relevant improvement in terms of clock speed is obtained LENGTH TCK3 100 200 200 200 by shortening the delay of input- ( ps ) output traces of the exclusive-or TDAT 300 300 200 100 gate U5A (100107) from 300ps (~ 6cm) to 100ps (~ 2cm) and trying F max. sweep 218 235 239 245 to increase the delay of the clock path of U2A with respect the ( MHz ) other flip-flops in the chain. This fixed - - - 238.1 last adjustment is implemented Fig. 5: Summary of the limit frequency versus layout parameters: trace length has great utilizing the clock driver gate impact on performance U1A (100102) as delay for U2A and U2B clock inputs and operation (Fig. 4). within device package, output trace connecting the other flip-flops -2 V RPD7 RPD11 RPD13 75 75 U5D TXOR2 430 7 1 Vee 75 ohm TDAT 100 ps 75 ohm 6 24 100107FP 100 ps TXOR1 Vee Vee 75 ohm Vee 300 ps 300 RPD3 RPD5 RPD10 U2A 430 430 430 RPD14 100131FP TINT3 TINT4 15 14 TINT1 11 TINT2 7 24 11 11 -2 V U2B U3C U3B U4B 16 24 100131FP 75 ohm 6 100131FP 100131FP 75 ohm 24 100131FP 75 75 ohm 75 ohm 100 ps 200 ps 100 ps 200 ps RPD12 20 13 12 8 12 12 1 5 1 400 1 Vee RPD9 330 RPD4 RS4 RPD6 4 3 RPD8 4 3 2 2 2 330 330 430 330 68 Vee Vee Vee TCK2 75 ohm 200 ps TCK3 75 ohm 200 ps TD1 U6A TD 17 15 75 ohm 100 ps 100 DATA OUT TCK1 75 ohm 300 ps 18 16 DATA OUT 75 ohm 75 ohm 100 ps 200 200 ps RPD21 100102FP RPD15 Vee TD2 10K 330 RPD22 RSCK1 10K 47 Vee Vee U1A U7D 15 17 TCIN1 100114FP 500 50 ohm 200 ps TC1 1 8 CK IN 75 ohm 18 16 100102FP CK IN 100ps RPD9 330 600 2 7 330 Vee 50 ohm 200 ps TCIN2 RT1 RPD1 100 -2 V 330 TCK01 U1D TC2 1 7 75 ohm 100 ps CKOUT 75 ohm 100 ps CKOUT 2 8 75 ohm 100 ps RPD19 RPD20 100102FP RPD17 RPD18 TCK02 330 10K 330 330 Vee Vee Vee Vee Fig. 6: Optimized version of the 31-bit pseudo-random generator board (PN50PT) The behaviour differences and connector, while output 100, directly to the output of the between output data signals being left open for bus differential line receiver U7A V(200) and V(100) are applications, has a more (100114) (Fig.6). Minor speed caused by straightforward behaviour because improvements are also obtained by different termination schemes: it is terminated off the board modifying the type of terminations complementary output 200 has a (RTERM visible on Fig. 3). used on critical paths turning series pull down load within the board The limit frequency for the initial into parallel arrangements. and no termination resistor, so that design turns out to be about the waveform is affected by strong 200 MHz due to setup time 3 Copyright Piero Belforte
  • 4. RESULTS The final result is the PN5OPT pre-layout version which operates correctly up to about 238 MHz in the fixed frequency test and up to 245 MHz in the swept frequency set-up showing a 20% performance improvement over the initial design. This result is obtained with board implementations utilizing active devices packaged in flat-packs. It is interesting to evaluate the impact on performance due to -0.50 V * FP packaging effects: substituting larger 24-pin devices for the -1.30 V original flat-packs, while leaving V(100) layout4.g pcb trace lengths unchanged, * 245 MHz causes a decrease in maximum -2.00 V -0.50 V operating frequency (sweep test) * DIL from 245 MHz to 229 MHz showing the great importance of -1.30 V V(100) pn5db8dil.g packaging issues for this kind of * 229 MHz design (Fig. 7). An analysis -2.00 V -0.50 V procedure similar to that previously shown can be applied after the board layout definition in -1.30 V V(400) pn5db8dil.g order to verify the way in which the differences in the -2.00 V interconnection paths (length, 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 impedance, via holes, trace TIME[uS] proximity, etc.) can affect the Fig. 7: Maximum operating frequency (sweep test) comparison between flat and 24 pin dual-in-line package. performance with respect to the pre-layout specification. 4 Copyright Piero Belforte