GPIO & Memory-Mapped I/O

GPIO is the first peripheral you touch in nearly every piece of embedded firmware. It builds directly on the circuit fundamentals from Circuit Analysis Fundamentals — a GPIO pin is, electrically, just a controllable voltage source with current limits.

GPIO as memory-mapped registers

A microcontroller doesn't expose pins through a special instruction set — it exposes them as ordinary memory addresses. Writing to a register sets pin state; reading a register reads pin state. A simplified example, configuring and toggling a pin on an STM32-style part:

#define GPIOA_MODER   (*(volatile uint32_t *)0x40020000)
#define GPIOA_ODR     (*(volatile uint32_t *)0x40020014)
 
void led_init(void) {
    // Set pin 5 to general-purpose output mode (01)
    GPIOA_MODER &= ~(0b11 << (5 * 2));
    GPIOA_MODER |=  (0b01 << (5 * 2));
}
 
void led_toggle(void) {
    GPIOA_ODR ^= (1 << 5);
}

The volatile qualifier is not optional here: it tells the compiler this memory can change outside the normal flow of the program (or that writes have side effects), preventing it from optimizing away what looks like a "redundant" read or write.

GPIO alone only gets you simple, single-bit I/O. The next sub-lesson, Communication Protocols, covers how firmware talks to other chips over UART, SPI, and I2C.