main.c

// Copyright (c) 2022 Cesanta Software Limited
// All rights reserved

#include <inttypes.h>
#include <stdbool.h>

#define BIT(x) (1UL << (x))
#define PIN(bank, num) ((((bank) - 'A') << 8) | (num))
#define PINNO(pin) (pin & 255)
#define PINBANK(pin) (pin >> 8)

struct rcc {
  volatile uint32_t CR, PLLCFGR, CFGR, CIR, AHB1RSTR, AHB2RSTR, AHB3RSTR,
      RESERVED0, APB1RSTR, APB2RSTR, RESERVED1[2], AHB1ENR, AHB2ENR, AHB3ENR,
      RESERVED2, APB1ENR, APB2ENR, RESERVED3[2], AHB1LPENR, AHB2LPENR,
      AHB3LPENR, RESERVED4, APB1LPENR, APB2LPENR, RESERVED5[2], BDCR, CSR,
      RESERVED6[2], SSCGR, PLLI2SCFGR;
};
#define RCC ((struct rcc *) 0x40023800)

struct gpio {
  volatile uint32_t MODER, OTYPER, OSPEEDR, PUPDR, IDR, ODR, BSRR, LCKR, AFR[2];
};
#define GPIO(bank) ((struct gpio *) (0x40020000 + 0x400 * (bank)))

// Enum values are per datasheet: 0, 1, 2, 3
enum { GPIO_MODE_INPUT, GPIO_MODE_OUTPUT, GPIO_MODE_AF, GPIO_MODE_ANALOG };

static inline void gpio_set_mode(uint16_t pin, uint8_t mode) {
  struct gpio *gpio = GPIO(PINBANK(pin));  // GPIO bank
  int n = PINNO(pin);                      // Pin number
  gpio->MODER &= ~(3U << (n * 2));         // Clear existing setting
  gpio->MODER |= (mode & 3U) << (n * 2);   // Set new mode
}

static inline void gpio_write(uint16_t pin, bool val) {
  struct gpio *gpio = GPIO(PINBANK(pin));
  gpio->BSRR |= (1U << PINNO(pin)) << (val ? 0 : 16);
}

static inline void spin(volatile uint32_t count) {
  while (count--) asm("nop");
}

int main(void) {
  uint16_t led = PIN('B', 7);            // Blue LED
  RCC->AHB1ENR |= BIT(PINBANK(led));     // Enable GPIO clock for LED
  gpio_set_mode(led, GPIO_MODE_OUTPUT);  // Set blue LED to output mode
  for (;;) {
    gpio_write(led, true);
    spin(999999);
    gpio_write(led, false);
    spin(999999);
  }
  return 0;
}

// Startup code
__attribute__((naked, noreturn)) void _reset(void) {
  asm("ldr sp, = _estack");  // Set initial stack pointer

  // memset .bss to zero, and copy .data section to RAM region
  extern long _sbss, _ebss, _sdata, _edata, _sidata;
  for (long *src = &_sbss; src < &_ebss; src++) *src = 0;
  for (long *src = &_sdata, *dst = &_sidata; src < &_edata;) *src++ = *dst++;

  main();             // Call main()
  for (;;) (void) 0;  // Infinite loop in the case if main() returns
}

// 16 standard and 91 STM32-specific handlers
__attribute__((section(".vectors"))) void (*tab[16 + 91])(void) = {0, _reset};