Flow meter input capture

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

#define TIMER_HZ  (SystemCoreClock / 2)        /* APB1 clock */

static volatile uint16_t last;
static volatile uint32_t period;

void flow_init(void)
{
    TIM3->PSC = TIMER_HZ / 1000000u - 1u;
    TIM3->CCMR1 |= TIM_CCMR1_CC1S_0;       /* CC1 = input, mapped to TI1 */
    TIM3->CCER |= TIM_CCER_CC1E;           /* capture enabled, rising edge */
    TIM3->EGR = TIM_EGR_UG;                /* load the prescaler now */
    TIM3->DIER |= TIM_DIER_CC1IE;
    NVIC_EnableIRQ(TIM3_IRQn);
    TIM3->CR1 |= TIM_CR1_CEN;
}

void TIM3_IRQHandler(void)
{
    if (TIM3->SR & TIM_SR_CC1IF) {
        uint16_t now = (uint16_t)TIM3->CCR1;
        period = now - last;
        last = now;
    }
}

uint32_t flow_hz(void)
{
    return 1000000u / period;
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

Matched on the skills each company's embedded roles emphasise.

Related challenges