stm32f105 cortex-m3 getting to blink

This is not going to be a detailed post. All the code can be found in github at https://github.com/andete/blog-stuff/tree/master/STM32F105 .

In my quest for more experience with different microcontrollers I made a little breakout for the ST STM32F105 ARM Cortex-M3 micro-controller.

The breakout provides a 8Mhz external clock, which is multiplied by 9 by the internal PLL to give a nice 72Mhz clock.

The MCU is programmed with the STLINK V2 programmer via SWD. It should also be possible to program it via DFU but I have not tried that yet.

I cobbled some code together from all over the internet to make a simple blink file.

#include "stm32f10x.h"

// loosely based on
// http://www.overtracks.com/stm32/baby-steps.htm
// http://dics.voicecontrol.ro/dicsEE-IP/chapter/Setting%20GPIO%20manually
// http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0497a/BGBEEJHC.html
// https://forum.sparkfun.com/viewtopic.php?f=11&t=21107
// http://en.radzio.dxp.pl/stm32vldiscovery/

// LED is connected to PC0, so first bit on GPIOC
#define LED_BITMASK          1 << 0 // bit 0

#define USE_SYSTICK

#ifdef USE_SYSTICK
void SysTick_Handler(void) {
  GPIOC->ODR ^= LED_BITMASK; // toggle LED state
}
#else
void TIM3_IRQHandler(void) {
  // why the check for UIF ?
  if (TIM3->SR & TIM_SR_UIF) // if UIF flag is set
  {
    TIM3->SR &= ~TIM_SR_UIF; // clear UIF flag
    GPIOC->ODR ^= LED_BITMASK; // toggle LED state
  }
}
#endif

int main() {

  // enable GPIOC
  RCC->APB2ENR |= RCC_APB2ENR_IOPCEN;
  // configure GPIOC port 0 as output; 4 bits per pin
  // 4+0 -> (0) input mode, (4) floating (reset state)
  // 0+2 -> (2) output mode, (0) generic push-pull
  GPIOC->CRL = 0x44444442;

#ifdef USE_SYSTICK
  // 1 second = 72000000 / 8000000 = 9000000
  SysTick->LOAD = 9000000;
  // start counting from zero
  SysTick->VAL = 0;
  // enable and enable interrupt
  SysTick->CTRL |= (SysTick_CTRL_ENABLE_Msk | SysTick_CTRL_TICKINT_Msk);
#else
  // enable clock for timer3
  RCC->APB1ENR |= RCC_APB1ENR_TIM3EN;
  // Set prescaler to 24 000 (PSC + 1) // why?
  TIM3->PSC = 23999;
  // // Auto reload value 1000
  TIM3->ARR = 1000;
  // Enable update interrupt (timer level)
  TIM3->DIER = TIM_DIER_UIE; // Enable update interrupt (timer level)
  TIM3->CR1 = TIM_CR1_CEN;   // Enable timer

  NVIC_EnableIRQ(TIM3_IRQn);
#endif

  // sleep till the end of time
  while (1) { __WFI(); }
  return 0;
}

It does blinking via the SysTick interrupt or alternatively via the TIM3 timer interrupt. It is also possible to do this without interrupts at all, just using the timers, but this is not shown here.