FM-1-transporter

Development log

Hardware results, 2026-10-01. XIAO RP2040, three wires, no VBUS.

Item	Result
UBOOT entry through the PIO host	✅ handoff in ~1 ms, 4C4A:8057 mounted
Loader and chip info	✅ key 980F, flash 856014
1 MiB dump	✅ 3.3 s, identical to the V15 reference
Sector rewrite (0x92000)	✅ verified; V15 still boots
Out-of-range writes	✅ refused (0x0, 0x3000, 0x93000)
Soft key from V15	✅ UBOOT in ~1 s
USB_KEY after a watchdog reset	✅ ACK within 5 ms

Lessons

• CFG_TUH_API_EDPT_XFER must be 1. Without it, raw bulk transfers never complete.
• Keep the 1 ms pulses going until the host starts. Stock V15 needs them too.
• Without VBUS, stock V15 enables USB only after a cold power-on.
• The loader NAKs between packets. Speed came from 4 KiB reads plus NAK retries within each frame.

Logs: logs/

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