Timers

TIMER-Registers

• Timer Count Register (TIMx_CNT)

  Shows the current counter value. Size could be 32-bit or 16-bit depending on timer module used.

• Timer Auto-Reload Register (TIMx_ARR)

  Timer raises a flag and the counter restarts automatically when counter value reaches the value in the auto-reload register. The counter is an up counter by default but can also be configured to be a down counter.

  

• Timer Prescaler Register (TIMx_PSC)

  The prescaler slows down the counting speed of the timer by dividing the input clock of the timer

  

• Status Register (SR)

  

• Capture/Compare Register (ccr1, ccr2, ccr3, ccr4)

  

• Capture/Compare Mode Register

  

• Capture/Compare Enable Register (CCER)

  

TIMER- Some Terms

• Update Event

  When timeout occurs or how long it takes for flag to be raised

  

  The +1 is because we start from 0

• Period

  Value loaded into auto-reload register (TIM_ARR)

• Up counter

  Counts from zero to a set value.

• Down counter

  Counts from a set value down to zero

General Purpose Timer example

#include "stm32f4xx.h"

#define TIM2EN    (1U<<0)
#define CR1_CEN   (1U<<0)
#define SR_UIF    (1U<<0)

void tim2_1hz_init(void)
{
    /*Enable clock access to tim2*/
    RCC->APB1ENR |= TIM2EN;

    /*Set prescaler value*/
    TIM2->PSC = 1600 - 1;    // 16 000 000 / 1 600 = 10 000
    /*Set auto-reload value*/
    TIM2->ARR = 10000 - 1;   // 10 000 / 10 000 = 1
    /*Clear counter*/
    TIM2->CNT = 0;

    /*Enable timer*/
    TIM2->CR1 = CR1_CEN;
}

int main(void)
{
    tim2_1hz_init();

    while(1)
    {
        /*Wait for UIF */
        while(!(TIM2->SR & SR_UIF)){}

        /*Clear UIF*/
        TIM2->SR &= ~SR_UIF;

        printf("A second passed !! \n\r");
    }
}

Timer Output Compare example

#include "stm32f4xx.h"

#define TIM2EN      (1U<<0)
#define CR1_CEN     (1U<<0)
#define OC_TOGGLE   ((1U<<4) | (1U<<5))
#define CCER_CC1E   (1U<<0)
#define GPIOAEN     (1U<<0)
#define AFR5_TIM    (1U<<20)

void tim2_pa5_output_compare(void)
{
    /*Enable clock access to GPIOA*/
    RCC->AHB1ENR |= GPIOAEN;

    /*Set PA5 mode to alternate function*/
    GPIOA->MODER &= ~(1U<<10);
    GPIOA->MODER |= (1U<<11);

    /*Set PA5 alternate function type to TIM2_CH1 (AF01)*/
    GPIOA->AFR[0] |= AFR5_TIM;

    /*Enable clock access to tim2*/
    RCC->APB1ENR |= TIM2EN;

    /*Set prescaler value*/
    TIM2->PSC = 1600 - 1;   // 16 000 000 / 1 600 = 10 000

    /*Set auto-reload value*/
    TIM2->ARR = 10000 - 1;  // 10 000 / 10 000 = 1

    /*Set output compare toggle mode*/
    TIM2->CCMR1 = OC_TOGGLE;

    /*Enable tim2 ch1 in compare mode*/
    TIM2->CCER |= CCER_CC1E;

    /*Clear counter*/
    TIM2->CNT = 0;

    /*Enable timer*/
    TIM2->CR1 = CR1_CEN;
 }

int main(void)
{
	tim2_pa5_output_compare();

    while(1)
    {
    }
}

Timer Input Capture example

#include "stm32f4xx.h"

#define SR_CC1IF    (1U<<1)
#define TIM2EN      (1U<<0)
#define TIM3EN      (1U<<1)
#define CR1_CEN     (1U<<0)
#define OC_TOGGLE   ((1U<<4) | (1U<<5))
#define CCER_CC1E   (1U<<0)
#define GPIOAEN     (1U<<0)
#define AFR5_TIM    (1U<<20)
#define AFR6_TIM    (1U<<25)
#define CCER_CC1S   (1U<<0)

void tim3_pa6_input_capture(void)
{
    /*Enable clock access to GPIOA*/
    RCC->AHB1ENR |= GPIOAEN;

    /*Set PA6 mode to alternate function*/
    GPIOA->MODER &= ~(1U<<12);
    GPIOA->MODER |= (1U<<13);

    /*Set PA6 alternate function type to TIM3_CH1 (AF02)*/
    GPIOA->AFR[0] |= AFR6_TIM;

    /*Enable clock access to tim3*/
    RCC->APB1ENR |= TIM3EN;

    /*Set Prescaler*/
    TIM3->PSC = 16000 - 1; // 16 000 000 /16 000

    /*Set CH1 to input capture*/
    TIM3->CCMR1 = CCER_CC1S;

    /*Set CH1 to capture at rising edge*/
    TIM3->CCER = CCER_CC1E;

    /*Enable TIM3*/
    TIM3->CR1 = CR1_CEN;
}

void tim2_pa5_output_compare(void)
{
    /*Enable clock access to GPIOA*/
    RCC->AHB1ENR |= GPIOAEN;

    /*Set PA5 mode to alternate function*/
    GPIOA->MODER &= ~(1U<<10);
    GPIOA->MODER |= (1U<<11);

    /*Set PA5 alternate function type to TIM2_CH1 (AF01)*/
    GPIOA->AFR[0] |= AFR5_TIM;

    /*Enable clock access to tim2*/
    RCC->APB1ENR |= TIM2EN;

    /*Set prescaler value*/
    TIM2->PSC = 1600 - 1;   // 16 000 000 / 1 600 = 10 000

    /*Set auto-reload value*/
    TIM2->ARR = 10000 - 1;  // 10 000 / 10 000 = 1

    /*Set output compare toggle mode*/
    TIM2->CCMR1 = OC_TOGGLE;

    /*Enable tim2 ch1 in compare mode*/
    TIM2->CCER |= CCER_CC1E;

    /*Clear counter*/
    TIM2->CNT = 0;

    /*Enable timer*/
    TIM2->CR1 = CR1_CEN;
 }

int timestamp = 0 ;

/*Set up : Connect a jumper wire from PA5 to PA6*/

int main(void)
{
    tim2_pa5_output_compare();
    tim3_pa6_input_capture();

    while(1)
    {
        /*Wait until edge is captured*/
        while(!(TIM3->SR & SR_CC1IF)){}

        /*Read captured value*/
        timestamp = TIM3->CCR1;
    }
}
// ...existing code...