YMODEM XMODEM Integration
Overview
Use this skill for serial file-transfer protocols in bootloaders or maintenance consoles. Separate transport reliability, packet parsing, CRC/checksum, flash layout, and update safety before writing firmware to flash.
When To Use
Use this skill when:
- The user wants to add or debug XMODEM/YMODEM firmware update over UART, USB CDC, or shell.
- Transfers timeout, abort, fail CRC, write wrong file size, brick after update, or hang at
C/NAK negotiation. - The task involves bootloader app download, serial protocol state machines, packet size, CRC16, SOH/STX/EOT/CAN, or flash programming.
Do not use this skill for general serial logging. Use embedded-serial-log-debug first if RX/TX is unreliable.
First Questions
Ask for:
- Protocol: XMODEM, XMODEM-CRC, XMODEM-1K, YMODEM, or YMODEM batch.
- Transport, baud, flow control, timeout, and terminal sender tool.
- Bootloader/application flash layout and maximum image size.
- CRC/checksum mode and packet size.
- Whether current firmware/recovery image must be preserved.
- Current transfer log or packet trace.
Integration Checklist
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Prove serial transport. Verify raw RX/TX at the target baud before protocol debugging.
-
Implement protocol states explicitly. Negotiation, header, data packets, ACK/NAK, EOT, CAN abort, retry limit, and timeout should be visible in logs.
-
Validate file metadata. For YMODEM, parse filename and size. Reject images larger than the allowed slot.
-
Write flash safely. Erase only the intended slot, program aligned chunks, verify CRC/hash, and preserve bootloader/recovery areas.
-
Commit update last. Mark the image bootable only after full transfer and verify succeed.
-
Test abort and power loss. Ensure failed transfers do not leave the product without a bootable image.
Common Failures
- Sender uses checksum mode while receiver expects CRC mode.
- Packet sequence number or complement validation is skipped.
- 1K packets overflow a small receive buffer.
- Flash erase starts before image size and slot are validated.
- EOT handling completes before final flash sync/verify.
- Failed transfer marks a bad image as bootable.
Verification
Before claiming transfer/update works:
- State protocol variant, sender tool, baud, packet size, timeout, and flash slot.
- Confirm successful transfer, CRC/hash, flash verify, and boot decision.
- Confirm oversize image rejection.
- Confirm cancel/timeout behavior.
- Confirm recovery path after interrupted transfer.
Example
User:
Bootloader 用 YMODEM 升级，传完后 app 起不来。
Agent:
- Asks for protocol variant, sender tool, app offset, image size, CRC, and boot log.
- Checks slot bounds and flash verify before app jump.
- Confirms image is marked bootable only after full verify succeeds.