What Pull-Up and Pull-Down Resistors Actually Do
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.
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