Shift Registers#

A shift register is a row of flip-flops wired so that each one’s output feeds the next one’s input. On every clock edge the whole row shifts its contents one position along. Where a plain register holds a word still, a shift register makes it move — which is the basis for nearly every way hardware moves data on a small number of wires.

Serial and Parallel, in Four Combinations#

The value of a shift register is converting between serial and parallel form, and parts are named for how data enters and leaves:

• SIPO (serial-in, parallel-out) — bits are clocked in one wire at a time, then read out all at once. This is the 74HC595, the standard way to turn a few pins into many outputs.
• PISO (parallel-in, serial-out) — a word is captured in parallel, then clocked out one bit at a time. This is the 74HC165, the mirror for reading many inputs on a few pins.
• SISO (serial-in, serial-out) delays a stream by N clocks; PIPO with shifting, and the universal shift register (74HC194) that does any direction on command, round out the set.

Serial-to-parallel and parallel-to-serial conversion is exactly what a UART or an SPI port does at its core — a shift register clocking bits onto or off of a wire.

Feedback Turns It Into More#

Tapping a shift register’s output back to its input changes what it is:

• Feed the last output straight back and it becomes a ring counter — a one-hot pattern circulating.
• Feed the inverted output back and it becomes a Johnson counter.
• Feed back an XOR of a few taps and it becomes a linear-feedback shift register (LFSR), cycling through a long pseudo-random sequence — the basis of scramblers, cheap noise sources, checksums, and test-pattern generators.

The same handful of flip-flops that move data in a line also, with a wire looped back, count, sequence, and generate pseudo-random patterns — which is why the shift register is one of the most reused structures in all of digital logic.