Shift Registers

A small chip that turns a handful of pins into many more, by loading output bits one at a time instead of all at once.

A microcontroller only has a limited number of GPIO pins, and it's easy to run out once a project needs to drive many individual LEDs or read many buttons. A shift register is a simple, cheap IC that expands a small number of pins into many more outputs (or inputs), by trading speed for pin count.

A common example is the 74HC595, an 8-bit "serial-in, parallel-out" shift register. Instead of wiring 8 separate output pins from your microcontroller, you use just 3: a data pin, a clock pin, and a latch pin. The microcontroller sends one bit of data at a time on the data pin, pulsing the clock pin after each bit — this "shifts" that bit into an internal 8-bit storage register inside the chip, one position at a time. Once all 8 bits have been shifted in, pulsing the latch pin makes all 8 of the chip's output pins update simultaneously to match what was just shifted in.

The name describes the mechanism exactly: bits shift, one place at a time, through a register (a small internal memory), like a line of people each passing a bucket to the next. Multiple shift registers can even be chained together — the last bit shifted out of one feeds directly into the next — letting a handful of microcontroller pins ultimately control dozens or hundreds of outputs.

The tradeoff is speed: shifting 8 bits in one at a time is much slower than setting 8 pins directly and simultaneously. For things like static LED displays or infrequently-updated indicators, that's a completely worthwhile trade for the pins saved.