GPIO

GPIO is the fundamental building block of embedded hardware interaction. In output mode, a pin is driven high or low by writing to a port data or set/clear register, which in turn sources or sinks current to control LEDs, drive logic signals, toggle chip-select lines, or generate bit-banged waveforms. In input mode, the pin state is sampled by reading a port input register, which lets firmware detect button presses, read digital sensor outputs, or sample external logic levels. Many MCU GPIO controllers support configurable internal pull-up or pull-down resistors, which can reduce the need for discrete resistors on floating inputs; however, internal pulls are not universal and are often weaker than external resistors, so noisy or timing-sensitive designs may still require discrete parts.

Output drive configuration matters in practice. Common options include push-pull (the pin actively drives both high and low) and open-drain (the pin can only pull low; an external resistor pulls the line high). Open-drain outputs are required or preferred on shared buses such as I2C and can be useful in some level-shifting circuits, though open-drain is one of several approaches for interfacing devices at different supply voltages rather than a universal requirement. Exceeding a pin's rated source/sink current -- typically 4 mA to 25 mA per pin depending on the device -- can cause voltage levels to fall outside valid logic thresholds or, in extreme cases, damage the pin driver.

On most modern MCUs, GPIO pins that share pads with alternate functions must be switched into the correct mode via a pin multiplexer or pin function select register before use. Forgetting to configure the alternate function, or leaving a pin in its default state after reset when a peripheral is expected to drive it, is a common source of bringup bugs. Interrupt-capable GPIO inputs, where a pin state change triggers an ISR rather than requiring polled reads, add further configuration options such as edge selection (rising, falling, or both) and debounce.

When running embedded Linux on an application processor (Cortex-A, RISC-V SoC, etc.), GPIO is accessed through the kernel's GPIO subsystem rather than by direct register writes. The modern, recommended interface is the character device API exposed by libgpiod, which replaces the older sysfs-based approach. Direct register access from userspace is generally not the recommended approach on Linux targets because it bypasses the kernel's resource management and can conflict with kernel drivers, though some specialized platforms and setups do use it -- a point covered in the blog post "Libgpiod - Toggling GPIOs The Right Way In Embedded Linux".