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UART firmware-update protocol

Simple, framed, request/response protocol between the host and the bootloader.

Serial settings

  • 115200 baud, 8 data bits, no parity, 1 stop bit (8N1)

Frame format

+--------+--------+-----------------+-----------+
|  SOF   |  CMD   |  LEN (2 bytes)  |  PAYLOAD   |  CRC32 (4 bytes)  |
| 0xA5   | 1 byte |  little-endian  |  LEN bytes |  over CMD..PAYLOAD |
+--------+--------+-----------------+-----------+
  • SOF — start of frame, always 0xA5
  • CMD — command byte (below)
  • LEN — payload length, little-endian uint16
  • PAYLOAD — command-specific
  • CRC32 — CRC-32 (IEEE 802.3, poly 0x04C11DB7) over CMD + LEN + PAYLOAD

Commands (host → bootloader)

CMD Name Payload Meaning
0x01 HELLO none probe; bootloader replies with version
0x02 ERASE none erase the application region (sectors 2–7)
0x03 WRITE offset:u32 + data write data at app_base + offset
0x04 VERIFY crc32:u32 verify CRC32 of the whole written image
0x05 JUMP none relocate VTOR and boot the application

Responses (bootloader → host)

Byte Name Meaning
0x79 ACK success (HELLO also returns 1 version byte)
0x1F NACK failure (bad CRC, out-of-range offset, flash error)

Notes

  • The host waits for an ACK/NACK after every frame before sending the next.
  • WRITE offsets must be word-aligned; the bootloader rejects unaligned or out-of-range writes.
  • The bootloader never erases its own region (sectors 0–1).