GPIO: General-Purpose Input/Output

The pins that let firmware touch the real world.

Each GPIO pin can be an input or an output. As an output it drives a high or low voltage, enough for a small LED through a resistor. As an input it reads whether the voltage is above or below a threshold.

A floating input picks up noise and reads randomly. Fix this with a pull-up or pull-down resistor, often built into the chip and enabled in software.

Pins have current limits, so drive motors and relays through a transistor, never directly.

/* hypothetical register names for illustration */
DIR_REG  |= (1u << 3);   /* pin 3 becomes an output */
OUT_REG  |= (1u << 3);   /* drive it high */
OUT_REG  &= ~(1u << 3);  /* drive it low */
if (IN_REG & (1u << 5)) { /* pin 5 reads high */ }