# 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:

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

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

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

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

## Quick validation

CDC path:

```bash
./.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`](../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:

```bash
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
