// LD3 connected to PB3

#include "stm32l0xx.h"

#define CLOCK_SPEED 2097000u
#define SYSTICK_LOAD (CLOCK_SPEED / 1000) - 1

volatile uint32_t msTicks = 0;

void SysTick_Handler(void) // called when SysTick Val reaches 0
{
  msTicks++;
}

void delay_ms(uint32_t ms)
{
  uint32_t start  = msTicks;

  while ((msTicks - start) < ms)
  {
    start = msTicks;
  };
}

int main(void)
{
  // SysTick initialization
  SysTick->LOAD = SYSTICK_LOAD;
  SysTick->VAL = 0u;
  SysTick->CTRL |= SysTick_CTRL_ENABLE_Msk | SysTick_CTRL_TICKINT_Msk | SysTick_CTRL_CLKSOURCE_Msk;

  // Turn on RCC for GPIOB
  RCC->IOPENR |= RCC_IOPENR_IOPBEN;

  // Set PB3 to output in MODER
  GPIOB->MODER &= ~(3 << 6);
  GPIOB->MODER |= (1 << 6);

  while (1)
  {
    // Toggle PB3 on/off
    GPIOB->ODR ^= (1 << 3);

    delay_ms(500);
  }

  return 0;
}