Periodic control loop

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 float integral;
static float prev_err;

float pid_step(float err, float dt)
{
    integral += err * dt;
    float out = KP * err + KI * integral
              + KD * (err - prev_err) / dt;
    return out;
}

void control_task(void *p)
{
    for (;;) {
        float err = setpoint - read_temp();
        set_output(pid_step(err, 0.01f));
        vTaskDelay(pdMS_TO_TICKS(10));
    }
}

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.

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