01 Overview

TWAI is Espressif's implementation of the CAN protocol, compatible with CAN 2.0 standard and extended frames. The official example demonstrates basic frame transmission and reception, serving as the starting point for board-level CAN communication.

It solves the problem of directly interfacing ESP32 series with a CAN bus, exchanging data with automotive ECUs, industrial PLCs, sensor gateways, etc., without needing an external SPI-to-CAN module.

Typical use cases: vehicle connectivity, industrial fieldbus, device status reporting, multi-node control networks.

It is not: a USB-to-CAN debug tool, nor a full high-layer protocol stack (like CANopen or J1939). It only provides low-level frame transmission; protocol parsing must be implemented by the user.

On the hardware side, the chip only contains a CAN controller; an external CAN transceiver (e.g., SN65HVD230) is mandatory to generate the differential signal.

02 Hardware

• Transceiver: An external CAN transceiver (e.g., SN65HVD230) is mandatory. Connect the chip's TWAI pins (e.g., GPIO5/GPIO4) to the transceiver's TXD/RXD.
• Bus termination: Connect a 120Ω resistor between CANH and CANL, one at each end of the bus.
• Common ground: All nodes must share a common ground; otherwise, common-mode voltage may damage the transceiver.
• Isolation: In noisy environments, use isolated transceivers (e.g., ISO1050) or isolated power supplies to avoid ground loops.
• Power: The transceiver typically requires 3.3V or 5V supply; ensure voltage level compatibility, add level shifting if necessary.
• Wiring: Use twisted pair for CANH/CANL, keep it short, and route away from high-power lines.
• Schematic: Refer to the schematic for specific pin connections and decoupling capacitors.

03 Software flow

1. Configure the TWAI driver: set bitrate (e.g., 500kbps), mode (normal/listen-only), acceptance filter (receive all frames or filter by ID).

2. Install the driver: call twai_driver_install() to initialize the controller.

3. Start TWAI: call twai_start() to begin communication.

4. Transmit frames: construct a twai_message_t, fill ID, data length, and data, then call twai_transmit().

5. Receive frames: call twai_receive() for blocking reception, or use event queues for asynchronous reception.

6. Error handling: monitor error counters (tx_err_counter/rx_err_counter); if they exceed thresholds, the controller enters bus-off state, requiring twai_initiate_recovery() to recover.

7. Stop and uninstall: twai_stop() and twai_driver_uninstall() are used to shut down.

04 Core points

• No transceiver, no CAN: Connecting the chip pins directly to the bus will damage the chip; an external transceiver is mandatory.
• Bit timing is sensitive: Different chips (e.g., S3/C3) have different clock sources, so bit timing parameters may vary; adjust according to the chip's datasheet.
• Acceptance filter: The default receives all frames; in real applications, configure filters to reduce CPU load.
• Bus-off recovery: After bus-off, manual recovery is required; otherwise, communication will not resume.
• Custom work: Bitrate, filters, and protocol layers (e.g., CANopen) need customization per project.
• Don't copy across chips: TWAI pins may differ across ESP32 series, and bit timing parameters must be adjusted per chip.