esp32_twai

TWAI (CAN 2.0): the ESP32’s CAN controller, an SJA1000-style core.

This driver covers the self-test loopback path: the controller transmits a frame and receives its own copy, with no second node and no bus.

How the loopback works, without a wire or a transceiver

Two mechanisms combine:

• Self-test mode (MODE.STM) lets a transmission complete with no acknowledgement — normally another node must ACK or the frame is retried forever. A self-reception request (CMD.SRR) transmits and receives the same frame at once.
• The transmitted bits still have to reach the RX input. Routed through the GPIO matrix, the controller’s TX output and RX input signals land on the same pad, so what it drives it reads back. Real analog path (pad → pad), no internal register loopback.

So a single ESP32, one pin, tests the whole controller: bit timing, framing, CRC, the TX and RX buffers.

Not covered

A real bus, arbitration against another node, and acknowledgement by a second controller. Those need a transceiver and a peer (issue #27). This is the on-chip functional test, fixed at 125 kbit/s — the rate is immaterial when a node talks only to itself.

Register facts

DR_REG_CAN_BASE = 0x3FF6B000, from esp-idf soc/soc.h. The register layout is soc/twai_struct.h (each 8-bit SJA1000 register in the low byte of a 32-bit word); the sequences are hal/twai_ll.h.

Register	Offset	Fields
MODE	0x00	RM 0, LOM 1, STM 2, AFM 3
CMD	0x04	TR 0, AT 1, RRB 2, CDO 3, SRR 4
STATUS	0x08	RBS 0, DOS 1, TBS 2, TCS 3, RS 4, TS 5, ES 6, BS 7
INT_ENA	0x10	interrupt enables, all cleared
BUS_TIMING_0	0x18	BRP [5:0], SJW [7:6]
BUS_TIMING_1	0x1C	TSEG1 [3:0], TSEG2 [6:4], SAM 7
DATA[0..13]	0x40..0x74	TX/RX buffer; ACR/AMR in reset mode
CLOCK_DIVIDER	0x7C	CD [2:0], CO 3, CM 7 (PeliCAN)

Structs

Frame

struct Frame {
    pub id: u16,
    pub len: u8,
    pub data: [u8; 8],
}

A standard (11-bit identifier) CAN frame.

Implements: Clone, Copy, Debug, Eq, PartialEq, StructuralPartialEq

Twai

struct Twai { /* private fields */ }

The TWAI controller.

Enums

Mode

enum Mode {
    Normal,
    SelfTest,
    ListenOnly,
}

Operating mode.

Implements: Clone, Copy, Debug, Eq, PartialEq, StructuralPartialEq

TwaiError

enum TwaiError {
    Timeout,
}

Why a self-reception failed.

Implements: Clone, Copy, Debug, Eq, From, PartialEq, StructuralPartialEq