Correct EEPROM soft-brick record: partial write on undersized U7 kills enumeration
Hardware bring-up correction (Dennis, 2026-08-22): writing the FT2232H product string to the undersized 93LC46B (U7) does NOT harmlessly fail — the partial, mirrored write lands a checksum-valid-but-garbage config, and the FT2232H then FAILS USB enumeration entirely (dead silent, no dmesg attach). It does NOT fall back to ROM defaults; only a blank/checksum-invalid EEPROM does. The earlier claim that "a bad EEPROM always falls back and still enumerates" was WRONG. Recovery (confirmed working): power on with U7 CLK shorted to GND so the FT2232H can't read a valid config -> forces ROM defaults -> enumerates -> then erase U7. Updated REVIEW.md, TODO.md, the flash-re-bba-rb skill (do-not-program warning + recovery, native-Linux troubleshooting), and flash_ftdi_eeprom.py comments/help to state the soft-brick reality. The script already refuses the write on mirroring detection; --force now documented as "reproduce the soft-brick". Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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@@ -5,32 +5,35 @@ Uses pyftdi (libusb) — the in-container tool, since debian's libftdi1-2 ships
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only the runtime lib, not the `ftdi_eeprom` binary. The intended descriptor
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values mirror hardware/re-bba-rb/ftdi_eeprom.conf.
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SAFE BY DESIGN:
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* Keeps the stock VID/PID 0403:6010 and the dual-channel config, so iceprog
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(MPSSE on channel A) and the channel-B UART keep working unchanged — only
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the descriptor STRINGS change.
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* DRY-RUN BY DEFAULT: prints the diff and writes a raw backup, but does NOT
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touch the EEPROM unless you pass --commit.
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* Always dumps the current EEPROM to a backup file first.
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DRY-RUN BY DEFAULT: prints the diff and writes a raw backup, but does NOT touch
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the EEPROM unless you pass --commit. Keeps the stock VID/PID 0403:6010.
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Usage (inside the rebuilt devcontainer, FT2232H usbipd-attached):
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Usage (inside the devcontainer, FT2232H attached):
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sudo /opt/venv/bin/python hardware/re-bba-rb/flash_ftdi_eeprom.py # dry-run + backup
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sudo /opt/venv/bin/python hardware/re-bba-rb/flash_ftdi_eeprom.py --commit # write strings
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sudo /opt/venv/bin/python hardware/re-bba-rb/flash_ftdi_eeprom.py --erase --commit # blank U7
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⚠️ re-bba-rb V1 HARDWARE LIMITATION: U7 is a 93LC46B (128-byte / 1 Kbit) EEPROM,
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which is TOO SMALL for the FT2232H — the H-series needs a 93LC56B (256 B) or
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93LC66B (the 93LC46 is for the FT232R / FT2232D). The FT2232H config mirrors in
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the 128-byte chip and the string write fails verify, so the "re-BBA-rb" product
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string CANNOT be programmed on V1. This script therefore REFUSES the write when
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it detects mirroring (use --force to override, --erase to blank U7). The board
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is fully functional without it: it enumerates on ROM defaults ("Dual RS232-HS",
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0403:6010) and iceprog + the channel-B UART work normally. Fix in a future rev
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by fitting a 93LC56B at U7.
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⚠️⚠️ re-bba-rb V1: DO NOT PROGRAM THE STRINGS — IT SOFT-BRICKS THE FT2232H. ⚠️⚠️
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U7 is a 93LC46B (128-byte / 1 Kbit) EEPROM, TOO SMALL for the FT2232H — the
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H-series needs a 93LC56B (256 B) or 93LC66B (the 93LC46 is for the FT232R /
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FT2232D). The FT2232H config mirrors in the 128-byte chip, so a string write
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lands a PARTIAL, checksum-valid-but-garbage config. The FT2232H then reads it at
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power-up and **FAILS USB ENUMERATION ENTIRELY** — dead silent, no dmesg attach.
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It does NOT fall back to ROM defaults (only a BLANK / checksum-INVALID EEPROM
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does that). This was confirmed on hardware 2026-08-22. The script therefore
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REFUSES the write when it detects mirroring; --force exists only to reproduce the
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soft-brick deliberately.
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Flash the FPGA bitstream FIRST and confirm iceprog works — the string is purely
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cosmetic. Do NOT rely on reset_device(): it re-enumerates the FT2232H and drops
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the usbipd attachment on WSL2 — re-attach with usbipd instead.
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RECOVERY if U7 ever gets a bad config written (done on the V1 unit, works):
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1. Power the board on with U7's CLK pin shorted to GND — the FT2232H then can't
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read a valid EEPROM and enumerates on ROM defaults ("Dual RS232-HS",
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0403:6010).
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2. Release the short and immediately blank U7:
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sudo /opt/venv/bin/python hardware/re-bba-rb/flash_ftdi_eeprom.py --erase --commit
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(Classic FTDI EEPROM recovery.) With U7 blank the board is fully functional;
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iceprog + the channel-B UART work normally. Fix for a future rev: 93LC56B at U7.
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reset_device() is intentionally NOT called — it re-enumerates the FT2232H and can
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drop a usbip attachment; re-plug / re-attach manually instead.
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"""
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import argparse
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import sys
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@@ -55,7 +58,8 @@ def main() -> int:
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help="blank U7 to 0xFF (FT2232H falls back to ROM defaults)")
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ap.add_argument("--force", action="store_true",
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help="attempt the string write even if U7 reports mirroring "
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"(undersized 93LC46B) — will likely fail/corrupt")
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"(undersized 93LC46B) — WILL SOFT-BRICK the FT2232H on "
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"V1 (fails USB enumeration; recover via CLK-to-GND + erase)")
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ap.add_argument("--url", default=URL, help=f"pyftdi device URL (default {URL})")
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args = ap.parse_args()
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@@ -94,17 +98,19 @@ def main() -> int:
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# 128 bytes: pyftdi sees the 128-byte chip mirror into the upper half
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# (has_mirroring=True), the config write mirror-clobbers word 0x80->0x00,
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# and verify fails. FTDI specs a 93LC56B (256 B) / 93LC66B for the H-series
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# (the 93LC46 is for the FT232R / FT2232D). So the product string CANNOT be
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# stored on V1 — the board still works fine on ROM defaults ("Dual RS232-HS",
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# 0403:6010; iceprog + the UART are unaffected). Refuse rather than
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# half-write (which corrupts U7 and needs an --erase recovery).
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# (the 93LC46 is for the FT232R / FT2232D). Writing the strings leaves a
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# PARTIAL checksum-valid-but-garbage config that SOFT-BRICKS the FT2232H: it
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# then fails USB enumeration entirely (dead silent — does NOT fall back to
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# ROM defaults). Recovery: power on with U7 CLK shorted to GND, then --erase.
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# So REFUSE the write — do not half-brick the chip.
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# ------------------------------------------------------------------
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if eeprom.has_mirroring and not args.force:
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print("\nREFUSING: U7 reports EEPROM mirroring — it is a 128-byte chip "
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"(93LC46B), too small for the FT2232H's 256-byte config.\n"
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"The 're-BBA-rb' product string cannot be stored on this board "
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"(see the header comment / REVIEW.md). The board works fine on ROM "
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"defaults. Use --erase to (re)blank U7; --force to attempt anyway.",
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"Writing the 're-BBA-rb' strings SOFT-BRICKS the FT2232H on this "
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"board (partial config -> fails USB enumeration; recover by shorting "
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"U7 CLK to GND at boot then --erase; see header / REVIEW.md).\n"
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"Use --erase to blank U7; --force ONLY to reproduce the soft-brick.",
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file=sys.stderr)
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eeprom.close()
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return 3
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