2. Objetivos:
• Determinar la salida de un F-F RS NAND y un F-F
NOR dado un m, state the output of an RS NAND and
RS NOR.
• Given a clock signal, determine the PGT and NGT.
• Define “Edge Triggered” and “Level Triggered”.
• Draw a Clocked F/F with and “Edge Triggered”
clock input and a “Level Triggered” clock input.
3. LOGIC CIRCUITS
Logic circuits are classified into two groups:
Combinational logic circuits Logic gates make decisions
Basic building
blocks include:
Sequential logic circuits Flip Flops have memory
Basic building blocks
include FLIP-FLOPS:
4. FLIP-FLOPS
•Memory device capable of storing one
bit
S Q •Memory means circuit remains in one
state after condition that caused the
state is removed.
R Q •Two outputs designated Q and Q-Not
that are always opposite or
complimentary.
•When referring to the state of a flip flop,
referring to the state of the Q output.
5. FLIP-FLOPS
Symbol
SET •To SET a flip flop means to
S Q make Q =1
RESET
R Q
Truth Table
•To RESET a flip flop means to
make Q = 0
6. FLIP-FLOPS
5V
+V
1k 1k
OUTPUT 1k 1k OUTPUT
Q NOT Q
1k 1k
NPN NPN
1k 1k
set reset
input input
•The flip flop is a bi-stable multivibrator; it has two stable states.
•The RS flip flop can be implemented with transistors.
7. R-S FLIP-FLOP
Symbols: Set Normal
S Q
FF
R Q
Reset Comple-
mentary
Truth Table:
Mode of Operation Inputs
Outputs
S R Q
Q’
Prohibited 0 0 1
1
Set 0 1 1 0
Reset 1 0 0 1
Hold 1 1 Q
Q’
8. R-S FLIP-FLOP
Active-Low
NAND LATCH
Q
SET
7400
Q NOT
RESET
7400
DEMORGANIZED NAND LATCH
NAND LATCH Q
SET
Q NOT
RESET
SET RES Q NOT-Q MODE
0 0 1 1 PROHIBITED
0 1 1 0 SET
1 0 0 1 RESET
1 1 NO CHG HOLD
12. TEST
Memory
1. Logic gates make decisions, flip flops have ____________________?
2. One flip flop can store how many bits? 1
3. What are the two outputs of a flip flop? Q Q-NOT
4. When referring to the state of a flip flop, we’re referring to the state
of which output? Q
5. What does it mean to SET a flip flop? Q = 1
6. What does it mean to RESET a flip flop? Q = 0
13. TEST
What is the mode of operation of the R-S flip-flop (set, reset or hold)?
What is the output at Q from the R-S flip-flop (active LOW inputs)?
L
? High
H
Mode of operation = ?
Set
H
? High
H
Mode of operation = ?
Hold
H
? Low
L
Mode of operation = ?
Reset
14. CLOCKED R-S FLIP-FLOP
Set FF Set FF
S Q S Q
Clock
CLK
Reset Q Reset Q
R R
ASYNCHRONOU SYNCHRONOUS
S
Outputs of logic circuit can Clock signal determines
change state anytime one exact time at which any
or more input changes output can change state
15. Clock
Digital signal in the form of a rectangular
or square wave
Astable
multivibrator
A clocked flip flop changes state only when
permitted by the clock signal
16. TRIGGERING OF FLIP-FLOPS
• Level-triggering is the transfer of data from input to
output of a flip-flop anytime the clock pulse is proper
voltage level.
• Edge-triggering is the transfer of data from input to
output of a flip-flop on the rising edge (L-to-H) or falling
edge (H-to-L) of the clock pulse. Edge triggering may be
either positive-edge (L-to-H) or negative-edge (H-to-L).
NGT-Negative Going Transition
PGT-Positive Going Transition
Negative-edge triggering
Positive-edge triggering
H
L
time
Level triggering
17. CLOCKED R-S FLIP-FLOP
Symbols: Set FF Normal
S Q
Clock
CLK
Reset Q
R Comple-
mentary
Truth Table:
Mode of operation Inputs
Outputs
Clk S R Q
Q’
Hold + pulse 0 0 no
change
Reset + pulse 0 1
0 1
Set + pulse 1 0 1 0
18. TEST
What is the mode of operation of the clocked R-S flip-flop (set, reset, hold)?
What is the output at Q from the clocked R-S flip-flop (active HIGH inputs)?
H
?
High
^
L Mode of operation = ? Set
L
?
High
^
Mode of operation = Hold
?
L
L
?Low
^
H
Mode of operation = ?
Reset
20. POSITIVE EDGE TRIGGERED
Symbols: R-S FLIP-FLOP
EDGE TRIGGERED R-S FLIP FLOP
SET Q
CLOCK
Q NOT
RESET
CLK SET RES Q NOT-Q MODE
PGT 0 0 NO CHG HOLD
PGT 0 1 0 1 RESET
Truth Table: PGT 1
PGT 1
0 1 0 SET
1 1 1 INVALID
CLK R S Q
0 X X NO CHG
1 X X NO CHG
X X NO CHG
0 0 NO CHG
0 1 SET
1 0 RESET
1 1 ILLEGAL
21. POSITIVE EDGE TRIGGERED
R-S FLIP-FLOP
TIMING DIAGRAMS
C
R
CLK R S Q
0
0
0
1
NO CHG
SET
S
1 0 RESET
1 1 ILLEGAL Q
22. NEGATIVE EDGE TRIGGERED
Symbols: R-S FLIP-FLOP
EDGE TRIGGERED R-S FLIP FLOP
SET Q
CLOCK
EDGE
DETECTOR
Q NOT
RESET
CLK SET RES Q NOT-Q MODE
PGT 0 0 NO CHG HOLD
PGT 0 1 0 1 RESET
Truth Table: PGT 1
PGT 1
0 1 0 SET
1 1 1 INVALID
CLK R S Q
0 X X NO CHG
1 X X NO CHG
X X NO CHG
0 0 NO CHG
0 1 SET
1 0 RESET
1 1 ILLEGAL
23. NEGATIVE EDGE TRIGGERED
R-S FLIP-FLOP
TIMING DIAGRAMS
C
R
CLK R S Q
0
0
0
1
NO CHG
SET
S
1 0 RESET
1 1 ILLEGAL Q
24. TEST
1. Type of flip flop where the outputs of circuit can change state anytime
one or more input changes? ASYNCHRONOUS
2. Type of flip flop where the clock signal controls when any output can
change state? SYNCHRONOUS
3. What do we call a digital signal in the form of a repetitive pulse or square wave?
CLOCK
4. Which is easier to design and troubleshoot, clocked or not clocked flip flops?
Clocked flip flops are easier to troubleshoot because we
can stop the clock and examine one set of input and
output conditions.
