🛠 Bus CAN FD : relier deux R4 avec un transceiver externe

Objectives

Move beyond point-to-point communication: the RA4M1 integrates a CAN FD (Flexible Data-rate) controller, a rare feature on an Arduino board — the official Arduino_CAN library exploits it. The RA4M1 provides the controller, but NOT the physical layer: you need an external CAN FD transceiver between the board and the bus. You will connect two boards and exchange frames at 500 kbit/s (arbitration) / 2 Mbit/s (data).

Hardware

• 2 Arduino UNO R4 WiFi
• 2 CAN FD transceiver modules (e.g., based on MCP2558FDT or TJA1051 for classic CAN to start — FD requires an FD-compatible transceiver)
• 120 Ω termination resistors at both ends of the bus (many modules include them, check the jumper)
• Wires: CANH↔CANH, CANL↔CANL (never crossed), grounds connected.

Code (transmitter, the other board as receiver with the twin example)

#include <CANFD.h>

void setup {
  Serial.begin(115200);
  // Débit d'arbitrage 500 kbit/s, débit de données 2 Mbit/s :
  if (!CANFD.begin(CanBitRate::BR_500k_BR_2M)) {
    Serial.println(F("CAN FD init KO"));
    while (true) { }
  }
}

void loop {
  static uint8_t compteur = 0;
  CanFrame trame;
  trame.canId = 0x123;
  trame.dataLength = 1;
  trame.data[0] = compteur++;
  CANFD.transmit(trame);
  Serial.print(F("emis ")); Serial.println(trame.data[0]);
  delay(500);
}

Expected Verification

On the receiver side, the frame arrives with its ID 0x123 and an incremented counter; the receiver board's LED matrix can display the value. Diagnostic test: disconnect CANL — nothing gets through anymore; remove one termination — intermittent arbitration errors.

Common Errors

• Connecting the controller directly to a bus without a transceiver: the RA4M1 does not have the physical layer; without a transceiver, absolutely nothing happens (and you can damage the pin).
• Using a classic CAN transceiver on an FD bus: the 2 Mbit/s data rate passes poorly or not at all; use an FD-compatible transceiver (or force both rates to be identical in CAN 2.0).
• Missing terminations or placed in the middle of the bus: both physical ends must have 120 Ω, not the middle node.
• CANH/CANL crossed on a single node: total silence — this is error #1, check them first.
• Confusing CAN FD and classic CAN regarding bit rates: arbitration and data can have two different bit rates; both nodes must be configured identically.