Shift register uses; 4-bit ring counter

Shift register and its applications

A shift register is a chain of flip-flops with a common clock that stores binary data and shifts it one position left or right at each clock pulse.

Applications:

• Temporary data storage (buffer registers).
• Serial-to-parallel and parallel-to-serial conversion (UART, serial communication).
• Time delay of serial data by n clock periods.
• Arithmetic: shift left/right to multiply or divide by 2.
• Ring counters and Johnson (twisted ring) counters, sequence generators.

4-bit ring counter

A ring counter is a shift register whose last output is connected back to the first input, so a single 1 circulates around the ring. A 4-bit ring counter has 4 states (mod-4, mod-n in general).

   +-----------------------------------------+
   |   PRE         CLR       CLR       CLR   |
   |  +-----+   +-----+   +-----+   +-----+  |
   +->|D  QA|-->|D  QB|-->|D  QC|-->|D  QD|--+
      |>    |   |>    |   |>    |   |>    |
      +-----+   +-----+   +-----+   +-----+
 CLK ---^---------^---------^---------^
 Start pulse: PRESET A, CLEAR B, C, D -> 1000

Working principle

1. A start pulse presets FF-A and clears FF-B, C, D, so QA QB QC QD = 1000.
2. At each rising clock edge every bit moves one place right and QD goes back to QA ($D_A = Q_D$).
3. The single 1 travels A to B to C to D and back to A, giving the sequence 1000, 0100, 0010, 0001, 1000, ...
4. Exactly one output is high at a time, so no decoder is needed; each output is high for one clock period out of every four.

Timing diagram

       1   2   3   4   5   6   7   8
CLK   _|‾|_|‾|_|‾|_|‾|_|‾|_|‾|_|‾|_|‾|
QA    ‾|___________|‾‾‾|___________|‾‾
QB    _|‾‾‾|___________|‾‾‾|__________
QC    _____|‾‾‾|___________|‾‾‾|______
QD    _________|‾‾‾|___________|‾‾‾|__

The frequency at each output is $f_{CLK}/4$. Ring counters are used as sequence (timing) generators and for controlling steps in sequential operations. The drawback is that n flip-flops give only n states.