# NFC (PN532) Debugging Guide

## Current Status
- **Hardware**: GPIO26(TX) → PN532 RXD, GPIO25(RX) ← PN532 TXD (UEXT pins 3-4)
- **DIP Switches**: Both OFF (HSU mode) ✓
- **Power**: UEXT pin 1 = 3.3V (verified), Pin 2 = GND (verified)
- **Wiring**: Confirmed correct
- **Problem**: PN532 module not responding to auto-baud probe

## Why NFC Probe Fails
The firmware attempts to communicate with PN532 at various baud rates:
- 115200, 9600, 57600, 38400 baud
- Tries both GPIO26/GPIO25 and GPIO25/GPIO26 (swapped)
- At each combination: sends `getFirmwareVersion()` command
- **Issue**: No response from PN532 → returns 0x00

## Prerequisites for Debugging
### 1. Enable Serial Output
**Arduino IDE Settings (REQUIRED):**
```
Tools → USB CDC On Boot: ENABLED
Tools → Port: /dev/ttyACM0
```

**Rebuild and flash the firmware:**
```bash
cd ~/arduino/olimex_ESP32-C5-EVB/esp32_arduino
arduino --verify --board esp32:esp32:esp32c5 esp32_arduino.ino
esptool.py --chip esp32c5 --port /dev/ttyACM0 --baud 921600 erase-flash
esptool.py --chip esp32c5 --port /dev/ttyACM0 --baud 921600 write-flash 0x0 build_output/esp32_arduino.ino.merged.bin
```

**Open Serial Monitor:**
- Arduino IDE → Tools → Serial Monitor
- Baud rate: **115200**
- Wait for boot messages

## Diagnostic Steps

### Step 1: Verify Serial Output
Expected output during boot:
```
=================================
ESP32-C5 Home Assistant Device
Arduino Framework
=================================
...
--- NFC (PN532 HSU) Initialization with Debug ---
Hardware: GPIO26(TX)→PN532-RX, GPIO25(RX)←PN532-TX, DIP:HSU mode
  [1/8] Baud=115200, RX=GPIO25, TX=GPIO26 ... ✗
  [2/8] Baud=9600, RX=GPIO25, TX=GPIO26 ... ✗
  ...
```

**If you see:**
- ✓ All "✗" symbols → PN532 not responding at any baud rate
- Nothing after "Initialization" → Serial output not working (re-check USB CDC setting)

### Step 2: Physical Verification Checklist
```
□ PN532 powered: Measure 3.3V on VCC pin (should be 3.0-3.3V)
□ PN532 GND: Verify continuity to board GND with multimeter
□ UEXT Pin 1: Should be 3.3V (measure with multimeter)
□ UEXT Pin 2: Should be GND (continuity check)
□ UEXT Pin 3 (GPIO26): No continuity to Pin 4 (should be separate signals)
□ UEXT Pin 4 (GPIO25): No continuity to Pin 3 (should be separate signals)
□ PN532 Module: Try re-seating in connector or using different PN532 board
```

### Step 3: Signal Analysis (Advanced)
If serial output shows all "✗" but hardware checks pass:

**Option A: Oscilloscope Check**
```
Connect oscilloscope probe to GPIO26 (TX line)
- Should see UART signal transitions during probe
- If flat line: ESP32 not transmitting
- If signal present: PN532 not responding
```

**Option B: Loopback Test**
```
Connect GPIO26 (TX) directly to GPIO25 (RX) with a jumper wire
Rebuild with loopback test code (at end of this file)
If loopback works: GPIO pins OK, PN532 module is issue
If loopback fails: GPIO configuration issue
```

## Possible Issues & Solutions

### Issue 1: PN532 Module Defective
**Symptoms:** All baud rates fail, hardware verified correct
**Solution:** 
- Try different PN532 board if available
- Test with I2C mode (different DIP setting) if supported
- Replace PN532 module

### Issue 2: UEXT Connector Loose
**Symptoms:** Intermittent detection or complete failure
**Solution:**
- Fully remove and re-seat PN532 in UEXT connector
- Ensure connector is fully inserted until it clicks
- Check for bent pins on UEXT connector

### Issue 3: Incorrect Pin Mapping
**Symptoms:** Detection works with swapped pins
**Solution:**
- Edit `esp32_arduino.ino`:
  - Find: `#define NFC_TX_PIN   26` and `#define NFC_RX_PIN   25`
  - If swapped RX/TX works: reverse these definitions
- Rebuild and test

### Issue 4: Baud Rate Mismatch
**Symptoms:** Serial output shows some attempts as "✓ FOUND!" but then fails to init
**Solution:**
- Firmware will re-init with confirmed baud rate
- If fails at SAMConfig() step: PN532 module may be in wrong mode

## Loopback Test Code
(If GPIO pins might be misconfigured):

```cpp
// Add to setup() after WiFi init, BEFORE NFC init:
void setup() {
    // ... existing code ...
    
    // LOOPBACK TEST
    Serial.println("--- GPIO Loopback Test ---");
    Serial.println("Jumper GPIO26 to GPIO25 to test UART");
    
    HardwareSerial testSerial(1);  // UART1
    testSerial.begin(115200, SERIAL_8N1, 25, 26);  // RX=GPIO25, TX=GPIO26
    
    delay(100);
    testSerial.print("TEST_LOOPBACK");
    delay(100);
    
    String received = "";
    while (testSerial.available()) {
        received += (char)testSerial.read();
    }
    
    if (received.indexOf("TEST") >= 0) {
        Serial.println("✓ GPIO26/GPIO25 loopback OK");
    } else {
        Serial.println("✗ GPIO loopback FAILED - pin configuration issue");
    }
}
```

## Next Steps
1. **Enable Serial Output**: Change USB CDC On Boot to ENABLED
2. **Rebuild & Flash**: Use updated Arduino IDE settings
3. **Monitor Serial**: Watch boot output during NFC probe
4. **Share Output**: Post the serial output here for analysis

**Expected Timeline:**
- Checking serial output: ~5 minutes
- Physical verification: ~10 minutes
- Analyzing results: ~5 minutes
- **Total**: ~20 minutes to identify root cause