Button debounce in an EXTI ISR

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.

volatile uint32_t presses;

void EXTI0_IRQHandler(void)
{
    presses++;
    HAL_Delay(20);
    EXTI->PR1 = EXTI_PR1_PIF0;
}

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