What the interviewer wants
- Watchdog architecture
- How each task proves it's alive and making progress
- What happens on a detected fault
Constraints
- STM32 with an independent watchdog (IWDG) and a window watchdog (WWDG)
- Motor control must never stop for more than 50 ms without the motor being put in a safe state
- Some tasks legitimately block for minutes (logging, BLE when idle)
- A reset must be logged with its cause
If you ask the interviewer
- Is there a separate safety processor? Not in this revision. Assume one MCU.
- What is the safe state for the motor? Motor driver disabled, alarm sounded. Disabling is done by a GPIO to the driver's enable pin.
- Can a reset happen during an infusion? Yes, but the system must come back in a safe state and report it.
How it's graded
The rubric a senior interviewer would use. Each item is scored 0 to 3 and weighted.
| Supervisor design | weight 3 |
| Liveness vs progress | weight 3 |
| Safe state and recovery | weight 3 |
| Diagnostics | weight 2 |
| Other hang sources | weight 1 |
| Validation | weight 2 |
How to structure your answer
- Requirements and assumptions: the numbers that drive the design.
- Architecture: blocks, tasks, data flow, memory.
- Key decisions and trade-offs, with the alternatives you rejected.
- Failure modes and recovery.
- Testing and validation.
More design rounds
Write your design in five sections and get it graded against this rubric, with a follow-up question from the AI interviewer.
Answer this round