GPIO

Many digital input problems come down to a simple cause: nobody is really driving the pin.

A button input toggles when it is not pressed. A relay glitches during power-up. An I2C edge rises too slowly. A long input wire false-triggers when a motor starts. Firmware sees only 0 and 1, but the physical node behaves like a random state.

The first job of a pull-up or pull-down resistor is to give a node a defined level when no active driver is controlling it.

Many embedded Linux device bugs look like register-access bugs: probe runs, register mapping succeeds, but reads return invalid values; interrupts never arrive; an I2C device randomly NACKs; the first access after resume fails.

The problem is not always in register access.

Whether hardware works often depends on more basic resources first: pins must be muxed correctly, clocks must be enabled, power must be stable, reset must be released, and GPIO polarity must be correct.