64-bit microsecond timestamps

The code

Embedded C for Cortex-M. It compiles. There are 2 real bugs in it: the kind that pass a bench test and fail in the field.

static volatile uint32_t overflow_count;

void TIM2_IRQHandler(void)
{
    if (TIM2->SR & TIM_SR_UIF) {
        TIM2->SR = ~TIM_SR_UIF;
        overflow_count++;
    }
}

uint64_t micros64(void)
{
    return ((uint64_t)overflow_count << 32) | TIM2->CNT;
}

static volatile uint64_t g_last_event_us;   /* written in an EXTI ISR */

uint64_t last_event_us(void)
{
    return g_last_event_us;
}

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