Tick timeouts and delays

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.

static volatile uint32_t g_ticks;   /* 1 ms SysTick */

void SysTick_Handler(void)
{
    g_ticks++;
}

bool timeout_expired(uint32_t start, uint32_t timeout_ms)
{
    return g_ticks > start + timeout_ms;
}

/* Busy-wait, roughly 4 cycles per loop */
void delay_us(uint16_t us)
{
    uint16_t loops = us * (SystemCoreClock / 1000000u) / 4u;
    while (loops--) {
        __NOP();
    }
}

/* Called from the main loop AND the button ISR */
void led_toggle(void)
{
    GPIOA->ODR ^= (1u << 5);
}

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

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