More Outputs#

A microcontroller or FPGA has run out of pins, but the design still needs to drive many things — a bank of LEDs, seven-segment digits, relays, or status lines. The trick is to trade a few pins and a little time for as many outputs as needed.

The Part#

The workhorse is the 74HC595, an 8-bit serial-in, parallel-out shift register with a latched output. Three signals — data, shift clock, and latch — feed eight bits in serially, and the latch then presents all eight at the outputs at once (so they change cleanly together rather than rippling). Its serial-output pin feeds the next ‘595’s input, so devices daisy-chain: three pins can drive 8, 16, 24, or more outputs, limited mainly by how long the shifting takes.

How It Fits Together#

The ‘595 is a shift register plus an output register — exactly the moving-data primitive, packaged for this job. Clock the bits in, pulse the latch, and the new pattern appears. Its mirror image, the 74HC165, solves the opposite problem of too few inputs.

Alternatives#

• An I²C or SPI GPIO expander (MCP23017, PCF8574) — adds a block of addressable pins on a bus, convenient when several are already on it.
• A dedicated LED driver (TLC5940, WS2812 strings) when the outputs are specifically LEDs needing constant-current or per-pixel control.
• Just use more pins — if a larger package or a second device frees enough I/O, the simplest answer is no expansion chip at all.