I2C Communication

Arduino Mega 2560 ↔ Raspberry Pi Pico I2C echo test with addressing, wiring, and troubleshooting.

Last updated: August 15, 2026

Pico I2C Configuration

Device Address:0x18(24 decimal)

Self Address:0x32(50 decimal, Pico's own)

Clock Rate:100 kHz

Test Mode:ECHO(mode=1) - echoes back all data received

Hardware Connections

Arduino Mega 2560          Raspberry Pi Pico
  ─────────────────          ─────────────────

  GND        ────────────────  GND
  3.3V*      ────────────────  3V3
  SDA (20)   ───┬──4.7kΩ──┬──  GPIO 4
                │         │
  SCL (21)   ───┴──4.7kΩ──┴──  GPIO 5

  *Note: Consider using a level shifter for 3.3V ↔ 5V conversion

4.7kΩ pull-up resistors are required on both SDA and SCL lines.

Arduino Mega Sketch

Key configuration for the Arduino sketch:

// I2C slave addresses (queried from Pico)
  uint8_t pico_target_address = 0x18;   // What Mega sends TO
  uint8_t pico_self_address = 0x32;     // Pico's own address (receives)

  // 100 kHz I2C clock on Arduino Mega
  #define I2C_CLOCK_100K 72  // TWBR register value

Upload Steps:

Open test_arduino_mega_i2c.ino in Arduino IDE
Go to Tools → Board → Arduino Mega 2560
Connect Arduino Mega via USB
Upload sketch

I2C Echo Test Sequence

1. Arduino → Pico (write):
     Address: 0x18 (24)
     Data: [0xA5, 0x5A]

  2. Pico processes (ECHO mode):
     Stores received bytes

  3. Arduino → Pico (read):
     Address: 0x18 (24)
     Expected: [0xA5, 0x5A]

  4. Arduino verifies:
     If read data == written data → TEST PASSED ✓

Expected Serial Output

========================================
  Arduino Mega 2560 - I2C/SPI/UART Test
  ========================================
  [I2C] Initialized at 100 kHz (TWBR=72)
  [SPI] Initialized
  [UART] Initialized (Serial1 @ 9600 baud)

  --- I2C TEST (Echo Mode) ---
  [I2C] Target Address: 0x18 (24)
  [I2C] Pico Self Address: 0x32 (50)

  [I2C] Writing 0xA5 0x5A to target device...
  [I2C] Write OK
  [I2C] Reading echo from target device...
  [I2C] Read OK: 0xA5 0x5A
  [I2C] ✓ ECHO VERIFIED - Data matches!

  --- Waiting 3 seconds before next test ---

Understanding I2C Addressing

device_address / target (0x18)

The address of the device the Pico will talk to. Usually another device on the I2C bus. In echo mode, this is where Mega sends data TO.

self_address (0x32)

Pico's own address on the I2C bus. Other masters use this to talk to the Pico. Must match Arduino's write target address.

In ECHO mode:

Arduino writes to 0x18 → Pico receives at 0x18 (self_address) → Pico echoes back
Arduino reads from 0x18 → Receives echo

API Commands

Query Pico I2C Config

python test_mega_pico_i2c.py

Set I2C to ECHO Mode

curl -X POST https://auth.ferqon.dev/api/pin/testmode \
      -H "Content-Type: application/json" \
      -d '{"gpio": 4, "mode": 1}'

Reconfigure Addresses

curl -X PATCH https://auth.ferqon.dev/api/pin/reconfigure \
      -H "Content-Type: application/json" \
      -d '{"gpio": 4, "device_address": 24, "self_address": 50}'

Troubleshooting

"Write ERROR: 2" (NACK on address)

Verify I2C clock rate is 100 kHz (TWBR=72)
Check SDA/SCL connections with 4.7k pullups
Make sure Pico address matches: 0x18

"Read ERROR (no data)"

Pico may not have received data yet
Add more delay between write and read
Check Pico is in ECHO mode

Data doesn't match (NACK_data error: 3)

Verify clock synchronization
Check wire integrity
Try lowering I2C speed (change TWBR value higher)

Key Points

Clock Rate: 100 kHz (standard for I2C)

Slave Address: 0x32 (50 decimal) - Pico receives here

Master (Mega): Sends to 0x18 (24 decimal)

Mode: ECHO - automatically echoes data back

Test Data: 0xA5 0x5A (165, 90 decimal)