Microsecond delays that hang

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.

void delay_us(uint32_t us)
{
    uint32_t start = DWT->CYCCNT;
    uint32_t cycles = us * (SystemCoreClock / 1000000u);
    while (DWT->CYCCNT < start + cycles) { }
}

void step_pulse(void)
{
    HAL_GPIO_WritePin(GPIOA, STEP_PIN, GPIO_PIN_SET);
    delay_us(2);
    HAL_GPIO_WritePin(GPIOA, STEP_PIN, GPIO_PIN_RESET);
}

void EXTI0_IRQHandler(void)          /* enable button */
{
    EXTI->PR1 = EXTI_PR1_PIF0;
    HAL_GPIO_WritePin(GPIOA, DRV_EN_PIN, GPIO_PIN_SET);
    HAL_Delay(5);
    step_pulse();
}

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.

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