Fixed-point speed 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.

#define KP_Q16  ((int32_t)(4.5 * 65536))
#define KI_Q16  ((int32_t)(0.8 * 65536))

static int32_t integ;

int16_t pi_step(int16_t target_rpm, int16_t rpm)
{
    int32_t err = target_rpm - rpm;
    integ += err;
    int32_t out = (KP_Q16 * err + KI_Q16 * integ) >> 16;
    return (int16_t)out;
}

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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