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#include "stm32f0xx.h"

static void prv_init_gpio(void) {
  // Enable GPIO clock - RM0091 6.4.6
  RCC->AHBENR |= RCC_AHBENR_GPIOCEN;

  // Set up GPIO as output - RM0091 8.3.10
  // Be sure to understand where these values are from and how they're calculated!
  // Understanding registers and how to read a datasheet is critical for debugging.

  // Set state first so we don't have a period where we're unsure of the output value
  // Use atomic bit set/reset register to reset the pin's output - RM0091 8.4.7
  // Prefer using BSRR over ODR - ODR requires read-modify-write
  GPIOC->BSRR = GPIO_BSRR_BR_9;

  // Mode: Output (0b01) - RM0091 8.4.1
  GPIOC->MODER &= GPIO_MODER_MODER9;
  GPIOC->MODER |= GPIO_MODER_MODER9_0;

  // Type: Push-pull (0b0) - RM0091 8.4.2
  GPIOC->OTYPER &= GPIO_OTYPER_OT_9;

  // Speed: Low (0b00) - RM0091 8.4.3
  GPIOC->OSPEEDR &= GPIO_OSPEEDER_OSPEEDR9;

  // Pull-up/pull-down: None (0b00) - RM0091 8.4.4
  GPIOC->PUPDR &= GPIO_PUPDR_PUPDR9;
}

static void prv_init_timer(void) {
  // Enable TIM2 clock - RM0091 6.4.5
  // Look at the clock tree to determine what clock peripherals are attached to (Fig 10, 11)
  RCC->APB1ENR |= RCC_APB1ENR_TIM2EN;

  // CR1: keep disabled (0b0), no clock division (0b0), up counter (0b0) - RM0091 18.4.2
  // See also 18.3.2
  TIM2->CR1 = 0;
  // Clock prescaler: Divider = PSC + 1 - RM0091 18.4.11
  // Assume TIM2 is connected to a 48MHz clock, divide by 48 to get a 1MHz clock
  TIM2->PSC = 48 - 1;
  // Auto reload: 0 to 999999 = 1000000 ticks - RM0091 18.4.12
  TIM2->ARR = 1000000 - 1;
  // Counter: reset to 0 - RM0091 18.4.10
  TIM2->CNT = 0;
  // Force an update event immediately - RM0091 18.4.6
  TIM2->EGR = TIM_EGR_UG;

  // Clear interrupt flag: Update (0b0) - RM0091 18.4.5
  // We clear the flag first so it isn't triggered by accident when we enable the interrupt source
  TIM2->SR &= ~TIM_SR_UIF;

  // Enable SYSCFG clock
  RCC->APB2ENR |= RCC_APB2ENR_SYSCFGEN;

  // Set NVIC priority to highest
  NVIC_EnableIRQ(TIM2_IRQn);
  NVIC_SetPriority(TIM2_IRQn, 0);

  // Enable TIM2
  TIM2->CR1 |= TIM_CR1_CEN;

  // Enable interrupt source: Update enabled (0b1) - RM0091 18.4.4
  // We enable it after enabling the timer clock to make sure we don't get an update event
  // from the forced update
  TIM2->DIER |= TIM_DIER_UIE;
}

int main(void) {
  prv_init_gpio();
  prv_init_timer();

  while (1) {
    __WFI();
  }

  return 0;
}

void TIM2_IRQHandler(void) {
  // TODO
  return 0;
} Toggle ODR value - Could also store a static variable or use BSRR
  GPIOC->ODR ^= GPIO_ODR_9;
  // Clear interrupt flags
  TIM2->SR = 0;
}