Feather S3 TFT Firmware

This directory contains the ESP-IDF firmware for the Feather S3 TFT signer target.

USB transport mode

Firmware now runs in TinyUSB composite mode (single USB peripheral owned by TinyUSB), exposing:

CDC-ACM interface for Python/serial tooling (/dev/ttyACM*)
Vendor interface for WebUSB browser transport

Expected runtime USB identity:

VID:PID 303a:4001
Product string: Feather S3 TFT n_signer

Flashing workflow (important)

Because TinyUSB owns USB at runtime, normal app mode is not the ROM USB-Serial/JTAG flashing interface.

Use this workflow for flashing:

Enter bootloader mode: hold BOOT, tap RESET, release BOOT.
Confirm bootloader USB device appears (typically /dev/ttyACM0).
Flash:

cd firmware/feather_s3_tft
idf.py -p /dev/ttyACM0 flash

Exit bootloader and run app: ensure BOOT is released, then tap RESET once.
Confirm app USB identity:

lsusb | grep -i 303a
# expect: 303a:4001 n_signer Feather S3 TFT n_signer

Quick validation

CDC path:

./.venv/bin/python ./examples/feather_get_public_key.py /dev/ttyACM0
./.venv/bin/python ./examples/feather_sign_event.py /dev/ttyACM0

WebUSB path:

Open examples/feather_webusb_demo.html
Connect in Chrome/Edge
Run get_public_key
Confirm pubkey matches CDC result

Linux WebUSB host setup (one-time)

Chrome and Edge need permission to open the device on Linux. Install a udev rule for the firmware VID:PID and reload rules:

sudo tee /etc/udev/rules.d/99-nsigner-webusb.rules >/dev/null <<'EOF'
SUBSYSTEM=="usb", ATTR{idVendor}=="303a", ATTR{idProduct}=="4001", MODE="0666", GROUP="plugdev", TAG+="uaccess"
EOF
sudo udevadm control --reload-rules
sudo udevadm trigger

After installing the rule, unplug and replug the device once, then reload the demo page.

Troubleshooting

If lsusb shows 303a:1001, the board is still in ROM bootloader mode. Tap RESET with BOOT released.
If CDC command hangs, verify app-mode VID:PID is 303a:4001, not 303a:1001.
If no /dev/ttyACM* appears, unplug/replug after reset and re-check lsusb.
If WebUSB connect throws SecurityError: Access denied, install the Linux udev rule above and replug.

CYD ESP32-2432S028 (CH340) serial-reset hardware note

For the CYD board (firmware/cyd_esp32_2432s028), opening /dev/ttyUSB0 can reset the ESP32 because CH340 DTR/RTS control lines are wired into the ESP32 auto-reset circuit.

Symptoms:

Device returns to startup/menu when a host app opens serial
Browser/CLI connect appears to "knock" signer out of ready screen

Field-proven mitigation:

Add a 10uF capacitor between EN and GND on the CYD board
A practical solder point is across the board reset path (e.g. reset button EN/GND pads)
For polarized electrolytic caps: negative leg to GND, positive to EN

Expected result after this hardware mod:

Serial open no longer triggers unwanted reset in normal host workflows
Signer remains in ready state across browser/CLI connect events

Notes:

Typical working range is ~4.7uF to 22uF; 10uF is recommended
Keep leads short for best stability
Auto-reset behavior for flashing may still work, but if flashing ever becomes unreliable, enter bootloader manually

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