stopwatch.c

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#pragma GCC optimize ("O2")

#include <stm32f4xx.h>
#include <hal.h>
/* 
 * Provides a micro-second granular delay using the CPU cycle counter.
 */

/* cycles per microsecond */
uint32_t us_ticks;

void stopwatch_init(void)
{
    us_ticks = SystemCoreClock / 1000000;
    
    /* turn on access to the DWT registers */
    DEMCR |= DEMCR_TRCENA; 
    /* enable the CPU cycle counter */
    DWT_CTRL |= CYCCNTENA;  

        stopwatch_reset();
}


void stopwatch_delay_us(uint32_t us){
//   we don't call stopwatch_reset() because any delay() in interrupt will reset main counter. It should be free running
    uint32_t ts = stopwatch_getticks(); // start time in ticks
    uint32_t dly = us * us_ticks;       // delay in ticks
    while(1) {
        uint32_t now = stopwatch_getticks(); // current time in ticks
        uint32_t dt = now - ts;

    if (dt >= dly)
        break;
    }
}