gpio-baremetal

GPIO (Bare-Metal)

Purpose

Configure and use GPIO without HAL: input/output modes, alternate function mapping, pull-up/down, speed/slew, and pin-change interrupts for LEDs, buttons, and peripheral pin mux.

When to Use

• Blink LED or read button before bringing up UART/SPI
• Mux pins to USART/SPI alternate functions
• EXTI line interrupts on pin edges
• Porting GPIO code between vendors

Workflow

1. STM32 GPIO pattern

/* Enable GPIOA clock */
RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;

/* PA5 output — MODER[11:10] = 01 */
GPIOA->MODER &= ~(3U << (5 * 2));
GPIOA->MODER |=  (1U << (5 * 2));

/* Toggle via BSRR — atomic set/reset */
GPIOA->BSRR = (1U << 5);       /* set */
GPIOA->BSRR = (1U << (5+16));  /* reset */

Alternate function (e.g., USART2 TX on PA2):

GPIOA->MODER &= ~(3U << (2*2));
GPIOA->MODER |=  (2U << (2*2));           /* AF mode */
GPIOA->AFR[0] &= ~(0xFU << (2*4));
GPIOA->AFR[0] |=  (7U << (2*4));          /* AF7 = USART2 */

2. Input with pull-up

/* PC13 input, pull-up */
GPIOC->MODER &= ~(3U << (13*2));          /* input mode 00 */
GPIOC->PUPDR &= ~(3U << (13*2));
GPIOC->PUPDR |=  (1U << (13*2));          /* pull-up */

int pressed = !(GPIOC->IDR & (1U << 13)); /* active low */

3. EXTI interrupt (STM32)

/* SYSCFG: map EXTI13 to PC13 */
SYSCFG->EXTICR[3] = (SYSCFG->EXTICR[3] & ~0xFU) | 0x2;  /* port C */
EXTI->IMR  |= (1U << 13);
EXTI->FTSR |= (1U << 13);  /* falling edge */
NVIC_EnableIRQ(EXTI15_10_IRQn);

ISR: check EXTI->PR, clear with write-1.

4. nRF52 / ESP32 notes

Platform	Pattern
nRF52	NRF_P0->PIN_CNF[n] — direction, pull, drive
ESP32	GPIO.out_w1ts, GPIO.enable; IO_MUX for function

Always enable peripheral clock / power domain before pin config.

5. Agent usage

/gpio-baremetal Configure PA2 as USART2 TX alternate function on STM32F4

Common Problems

Symptom	Cause	Fix
Pin stuck	Wrong MODER / not AF	Re-read RM pin table
EXTI storm	No debounce / floating input	Enable pull; debounce in software
AF mismatch	Wrong AFR nibble	Pin-specific AF table in RM
No toggle visible	Wrong port bit / LED active low	Check schematic

Related Skills

• skills/baremetal/mmio-and-bit-manipulation — BSRR/MODER masks
• skills/baremetal/peripherals-from-datasheet — RM pinout tables
• skills/baremetal/uart-serial-baremetal — AF mux for serial