CAN Bus Arbitration Experiment
ESP32 & BeagleBone Black

Software and Development Environment

The experiment consists of two independent programs developed in separate environments: PlatformIO (VSCode extension) for the ESP32, and a direct GCC compiler + PuTTY terminal for the BeagleBone Black.

# platformio.ini
[env:esp32dev]
platform = espressif32
board    = esp32dev
framework = arduino
monitor_speed = 115200

# Derleme
gcc -o can_arb_trigger \
    can_arb_trigger.c -lgpiod
echo "Exit: $?"

# Run
sudo ./can_arb_trigger can0

ESP32 Firmware — main.cpp

Receives trigger via hardware interrupt, waits sweepDelay µs, then transmits a TWAI frame with ID 0x456. On each trigger, TX/RX error counters and arb_lost_count are printed to Serial Monitor.

// ── Pin Definitions ────────────────────────────────────────
#define CAN_TX_PIN      GPIO_NUM_17   // SN65HVD230 TXD
#define CAN_RX_PIN      GPIO_NUM_16   // SN65HVD230 RXD
#define TRIGGER_PIN     GPIO_NUM_15   // BBB P9.15 → ESP32
#define MARKER_PIN      GPIO_NUM_4    // D0: timing marker

// ── Sweep Parameters ───────────────────────────────────────
#define DELAY_SWEEP_MIN   100         // µs
#define DELAY_SWEEP_MAX   170         // µs
#define DELAY_SWEEP_STEP  1           // her tetiklemede artar
#define CAN_ID_ESP32      0x456       // higher ID — loses

// ── ISR (Hardware Interrupt) ───────────────────────────────
void IRAM_ATTR onTriggerRising() {
    triggerFlag = true;
    triggerCount++;
}

// ── Main Loop ──────────────────────────────────────────────
void loop() {
    recoverFromBusOffIfNeeded();
    if (!triggerFlag) return;
    triggerFlag = false;

    delayMicroseconds(sweepDelay);    // timing adjustment

    // Marker HIGH → send frame → Marker LOW
    digitalWrite(MARKER_PIN, HIGH);
    twai_message_t msg = {.identifier = CAN_ID_ESP32,
                          .data_length_code = 8, .ss = 1};
    memset(msg.data, 0xEE, 8);
    twai_transmit(&msg, pdMS_TO_TICKS(10));
    digitalWrite(MARKER_PIN, LOW);

    // Advance sweep
    sweepDelay += DELAY_SWEEP_STEP;
    if (sweepDelay > DELAY_SWEEP_MAX) sweepDelay = DELAY_SWEEP_MIN;

    // Report results
    twai_status_info_t status;
    twai_get_status_info(&status);
    Serial.printf("delay=%uus | TX err:%lu | RX err:%lu | arb_lost:%lu\n",
        sweepDelay, status.tx_error_counter,
        status.rx_error_counter, status.arb_lost_count);
}

BBB Trigger Program — can_arb_trigger.c

Sends a trigger pulse from P9.15, then immediately writes a SocketCAN frame with ID 0x123. Reads the arbit-lost counter after each shot; stops when an increase is detected.

// ── Constants ─────────────────────────────────────────────
#define GPIO_CHIP        "/dev/gpiochip0"
#define GPIO_LINE        16    // P9.15 — tetik
#define GPIO_LINE_MARKER 17    // P9.23 — timing marker
#define CAN_ID_BBB       0x123 // lower ID — wins
#define TRIGGER_PULSE_US 50    // pulse width

// ── Main Loop (1000 shots, stop on arbitration) ────────────
while (shot < 1000) {
    shot++;

    // Trigger pulse: HIGH → 50µs → LOW
    gpiod_line_request_set_value(request, GPIO_LINE,
                                 GPIOD_LINE_VALUE_ACTIVE);
    sleep_us(TRIGGER_PULSE_US);
    gpiod_line_request_set_value(request, GPIO_LINE,
                                 GPIOD_LINE_VALUE_INACTIVE);

    // Marker + CAN frame
    gpiod_line_request_set_value(request, GPIO_LINE_MARKER,
                                 GPIOD_LINE_VALUE_ACTIVE);
    write(sock, &frame, sizeof(frame));
    gpiod_line_request_set_value(request, GPIO_LINE_MARKER,
                                 GPIOD_LINE_VALUE_INACTIVE);

    // arbit-lost check
    long current = get_arbit_lost(can_iface);
    if (current > arbit_before) {
        printf("*** ARBITRATION CAPTURED! +%ld ***\n",
               current - arbit_before);
        break;
    }
    sleep_ms(50);
}

CAN Interface Setup — BBB

The ESP32's TWAI driver uses a 75% sample point. BBB's default is 87.5%. A mismatch inflates the RX error counter — both sides must be set identically:

# Configure CAN interface to match ESP32
sudo ip link set can0 down
sudo ip link set can0 type can \
     bitrate 100000 \
     sample-point 0.75 \
     restart-ms 100
sudo ip link set can0 up

# Verify
ip -details link show can0 | grep sample
→ sample-point 0.750  ✓

# Arbitration statistics
ip -details -statistics link show can0 | grep -A1 re-started
→ re-started  bus-errors  arbit-lost  error-warn  error-pass  bus-off