What the interviewer wants
- Flash layout and boot flow
- Download, verification and switch-over
- What happens on power loss or a bad image at every step
Constraints
- 1 MB internal flash, 256 KB RAM; application image up to 380 KB
- Optional 4 MB external SPI NOR on the board
- Data is metered: a full image costs real money
- Battery can drop out at any moment (container shock, cold)
- Images must be signed; the company has an offline signing key
If you ask the interviewer
- Can we change the bootloader in the field? No. Assume the bootloader is fixed for the life of the product.
- How large are typical updates? Most are small fixes; a full image is the worst case.
- Is there a backend we control? Yes, an HTTPS server. It can serve byte ranges and keep per-device state.
- Is confidentiality of the image required? Integrity and authenticity are mandatory. Encryption is a nice-to-have.
How it's graded
The rubric a senior interviewer would use. Each item is scored 0 to 3 and weighted.
| Flash layout and boot flow | weight 3 |
| Authenticity and integrity | weight 3 |
| Atomic switch-over and rollback | weight 3 |
| Resumable, metered download | weight 2 |
| Power and battery awareness | 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