# n_signer Teensy 4.1 Firmware

**Status:** Planned — not yet implemented. See
[`plans/teensy41_signer_port.md`](../../plans/teensy41_signer_port.md).

The **Teensy 4.1** (NXP i.MX RT1062, Cortex-M7 @ 600 MHz) is the high-capacity
OTP pad target. Its built-in SD slot supports **1 TB SDXC cards (exFAT)** via
PJRC's SdFat library, making it the ideal hardware signer for large one-time-pad
storage.

## Why the Teensy 4.1

| Concern | Teensy 4.1 | CYD (ESP32) | Feather S3 TFT (ESP32-S3) |
|---|---|---|---|
| MCU | 600 MHz Cortex-M7 | 240 MHz Xtensa LX6 | 240 MHz Xtensa LX7 |
| SRAM | 1 MB + 16 MB PSRAM | 512 KB (no PSRAM) | 512 KB + quad PSRAM |
| SD slot | **4-bit SDMMC, exFAT, up to 2 TB** | 1-bit SDSPI, FAT32, up to 32 GB | — |
| USB | Hi-Speed (480 Mbps) device + host | CH340 UART only | Full-Speed USB |
| WiFi | **None** | Yes (unused) | Yes (unused) |
| Ethernet | 10/100 PHY (optional) | — | — |
| PQ crypto speed | ~1-2 s SLH-DSA-128s | 5-30 s SLH-DSA-128s | 5-30 s SLH-DSA-128s |

## SD card size support

| Card type | Size | Works? |
|---|---|---|
| SDSC | ≤ 2 GB | Yes (FAT16/32) |
| SDHC | 2 – 32 GB | Yes (FAT32) |
| **SDXC** | **32 GB – 2 TB** | **Yes (exFAT via SdFat)** |
| SDUC | 2 – 128 TB | No |

**1 TB SDXC cards work** — SdFat has native exFAT support, and the Teensy's
4-bit SDMMC bus runs at ~20-40 MB/s. No reformatting needed.

## Display + touch

**4.0" ST7796S 480×320 SPI TFT with XPT2046 resistive touch** (Hosyond or
equivalent, ~$12-15). Specs: 4-wire SPI, RGB 65K, 3.3V~5V (works at the Teensy's
3.3V logic), XPT2046 resistive touch, includes touch pen + on-module SD slot.

Resistive touch is the right choice for a hardware signer (deliberate physical
activation, stylus-compatible, simple driver). The XPT2046 driver ports from
the CYD's [`touch.c`](../cyd_esp32_2432s028/main/touch.c) with 480×320
resolution constants. The ST7796S display driver is new code (different init
sequence than the CYD's ILI9341).

Wiring: TFT SPI on pins 11/12/13, CS=10, DC=9, RESET=8, BL=22; touch shares
the SPI bus with T_CS=7, T_IRQ=6. See
[`plans/teensy41_signer_port.md`](../../plans/teensy41_signer_port.md) for the
full pin table.

## Build (planned)

```bash
# Arduino CLI / Teensyduino
arduino-cli compile --fqbn teensy:avr:teensy41 firmware/teensy41
arduino-cli upload -p /dev/ttyACM0 --fqbn teensy:avr:teensy41 firmware/teensy41
```

See the port plan: [`plans/teensy41_signer_port.md`](../../plans/teensy41_signer_port.md).