#include "stm32f30x.h"
    #include "system_stm32f30x.h"

    void TIM6_DAC_IRQHandler(void);
    void setTIM6(void);

volatile uint32_t _regValue = 1<<15;


    int main(void) {


    volatile int _timer = 1000000;

    RCC -> AHBENR |= (uint32_t) 1 <<21;
    GPIOE -> MODER |= 0x55550000; //ports 8-15 output
    setTIM6();
    TIM6 -> CR1 |=0x1; //timer enable

    while(1)
    {
    }
    } //end of main

    void setTIM6(void) {

    NVIC ->ISER[1] |= (uint32_t) 1<<22; //TIM6 int. enable
    RCC -> APB1ENR |= (uint32_t) 1<<4; //clock enable
    RCC -> CFGR |= 0x00000700; //APB1 prescaler -> clock=(8MHz/16)*2 = 1MHz
    TIM6->CR1 |= 0;
    TIM6->DIER |=1; //int. enable
    TIM6->PSC = 0xFFFF; //prescaler for TIM6. -> clk = 1Mz/2^16 = 2^4 =16Hz
    TIM6->ARR = 240; // int. flag every couple of secs.

      DBGMCU->APB1FZ |= 1<<4; //for debug - TIM6 counter freeze during breakpoint
}

void TIM6_DAC_IRQHandler(void)
{
    _regValue = _regValue >> 1;
    GPIOE -> ODR = _regValue; //writing value to PORTE output

    if (_regValue == 1<<9){
        _regValue = 1<<15;
    } //this is for rotational loop of LEDs

    TIM6->SR &= ~0x1; //UIF (Interrupt flag) disabled
}