Shift registers can be used for converting serial data to parallel form and for delaying digital signals. An 8-bit serial-in serial-out shift register with a 40 MHz clock can delay a signal by 200 ns as the delay for each clock cycle is 25 ns and there are 8 cycles. Shift-register counters include ring counters where the output of the last flip-flop is fed to the input of the first flip-flop, and Johnson counters where the inverted output of the last flip-flop is connected to the first flip-flop, allowing them to require fewer flip-flops than ring counters for a given count.

1. Shift Register Applications
‰Conversion of serial data to parallel form.
Sequential Logic
For example, assume the binary number 1011 is loaded sequentially, one bit
at each clock pulse.
After 4 clock pulses, the data is available at the parallel output.
FF0 FF1 FF2 FF3
Serial X 1 1 0 1data D0 Q0 D1 Q1 D2 Q2 D3 Q3
input
C C C C
CLK
108

2. Shift Register Applications
‰Delay a digital signal
Shift registers can be used to delay a digital signal by a predetermined amount.
EXAMPLE:
Sequential Logic
An 8-bit serial in/serial out shift register has a 40 MHz clock. What is the total delay through
the register?
The delay for each clock is
1/40 MHz = 25 ns
The total delay is 8 x
25 ns = 200 ns
Data in
CLK
40 MHz
CLK
Data in
A SRG 8 Q7
B
Q7
C
Data out
Data out
td
109

3. SHIFT-REGISTER COUNTERS Sequential Logic
Two types: ring counter and the Johnson (or shift) counter.
‰Ring Counter
The simplest shift-register counter is essentially a circulating shift register connected so that
the last FF shifts its value into the first FF. It is used to control the sequencing of operations
in a system.
(NOTE: The register need to be initialized to 1000)
In general, a ring
counter requires more
FFs than a binary
counter for the same
MOD number (MOD-N
requires N flip-flops)
110

4. Sequential Logic
SHIFT-REGISTER COUNTERS
‰Johnson Counter
like a normal ring counter except that the inverted output of the last FF is connected to the
input of the first FF.
For a given MOD number, a Johnson
counter requires only half the number
of FFs that a ring counter requires, but
generally more than a binary counter.
MOD-6 Johnson counter
111