# Programming Header Pinout

## Header J2 - 2x4 Programming Header (8 Pins)

Use this header to connect an external USB-to-Serial adapter (CP2102, CH340, FT232, etc.) for flashing the ESP32.

### Pinout (Top View)

```
┌─────────────┐
│ 1  2  3  4  │  ← Top row
│ 5  6  7  8  │  ← Bottom row
└─────────────┘
```

| Pin | Signal | ESP32 Connection | Description |
|-----|--------|------------------|-------------|
| 1 | +3V3 | +3V3 Power Rail | Optional - Power ESP32 from programmer |
| 2 | GND | GND | Ground reference |
| 3 | UART_TX | GPIO1 (UART TX) | ESP32 transmits to programmer |
| 4 | UART_RX | GPIO3 (UART RX) | ESP32 receives from programmer |
| 5 | DTR | GPIO0 | Data Terminal Ready (auto-reset for flashing) |
| 6 | RTS | EN | Request To Send (auto-reset for flashing) |
| 7 | NC | - | Not connected (spare) |
| 8 | NC | - | Not connected (spare) |

### Connection to USB-to-Serial Adapter

When connecting your USB-to-Serial adapter:

| Adapter Pin | J2 Pin | Signal |
|-------------|--------|--------|
| VCC | Pin 1 | +3V3 (optional - only if powering ESP32 from adapter) |
| GND | Pin 2 | GND |
| RX | Pin 3 | UART_TX (receives TX from ESP32) |
| TX | Pin 4 | UART_RX (sends to RX of ESP32) |
| DTR | Pin 5 | DTR (auto-reset) |
| RTS | Pin 6 | RTS (auto-reset) |
| - | Pin 7 | NC (spare) |
| - | Pin 8 | NC (spare) |

**Important Notes:**
- **Do NOT connect VCC** if your ESP32 is already powered from J1 (JST connector power input)
- **Always connect GND** - this is critical for proper operation
- **RX/TX are swapped** - Adapter RX goes to ESP32 TX, and vice versa
- **DTR and RTS** enable automatic reset during flashing (no need to press buttons)

### Optional Pull-up Resistors

For maximum reliability, add pull-up resistors:
- **R5**: 10kΩ between J2 Pin 5 (DTR) and +3V3
- **R6**: 10kΩ between J2 Pin 6 (RTS) and +3V3

Most USB-to-Serial adapters have built-in pull-ups, but adding them on the board ensures reliable operation.

### Flashing Process

1. Connect USB-to-Serial adapter to J2 header
2. Power the board (via J1 JST connector or J2 Pin 1)
3. Use `esptool` or ESPHome to flash:
   ```bash
   esptool.py --port /dev/ttyUSB0 write_flash 0x0 firmware.bin
   ```
4. The DTR/RTS signals will automatically put ESP32 into boot mode

### Common USB-to-Serial Adapters

- **CP2102**: Common, reliable, good driver support
- **CH340**: Very common, cheap, good for Linux
- **FT232**: Premium option, excellent driver support
- **PL2303**: Older, less reliable

All of these will work with this header configuration.