SANPO SPI to CAN

SANPO SPI to CAN lets a Jetson, Raspberry Pi, or another host controller operate classic CAN devices periodically. The host sends a fixed-length SPI message containing a CAN ID and data and reads device feedback during the current or a later transfer.

For CAN FD, see SANPO SPI to FDCAN.

SPI Settings

Item	Setting
SPI mode	Mode 0 (CPOL=0, CPHA=0)
Bit order	MSB First
SHORT	22-byte message area + 1-byte CRC, 23 bytes total
LONG	72-byte message area + 1-byte CRC, 73 bytes total
CRC-8	Polynomial `0x07`, initial value `0x00`, no final XOR

Factory default is SHORT. Classic CAN works in both SHORT and LONG, but use SHORT for lower overhead when CAN FD is not required.

Switch modes over USB serial:

AT+SPIMODE=SHORT
AT+SPIMODE=LONG

Send the command as text with a CRLF line ending. The setting is saved. After success, wait at least 10 ms before starting transfers with the new length.

Select a CAN Channel

SPI chip select	Channel	Target
First MCU	`0x01`	CAN-1
First MCU	`0x02`	CAN-2
Second MCU	`0x03`	CAN-3
Second MCU	`0x04`	CAN-4
Either chip select	`0x00`	Both CAN interfaces on the current MCU

Internally, 1/3 select the first local interface and 2/4 select the second. Replies only use local Channel 1/2.

Software CS0 and CS1 on a Linux host normally connect to the board’s two external chip selects. Confirm actual wiring rather than relying only on software numbering.

SHORT Message

Extended Frame

Byte position	Content
0 to 1	Fixed `45 54` (`ET`)
2	Channel
3 to 6	29-bit CAN ID, most significant byte first
7	Data length `0` to `8`
8 to 15	8-byte data area; fill unused bytes with `00`
16 to 21	`00`
22	CRC-8 over the first 22 bytes

Standard Frame

Byte position	Content
0 to 1	Fixed `53 54` (`ST`)
2	Channel
3 to 4	Fixed `00 00`
5 to 6	11-bit CAN ID, most significant byte first
7	Data length `0` to `8`
8 to 15	8-byte data area; fill unused bytes with `00`
16 to 21	`00`
22	CRC-8 over the first 22 bytes

For Channel 1, extended ID 0x0000FD01, and 8 data bytes, the 22 bytes before CRC are:

45 54 01 00 00 FD 01 08 01 02 03 04 05 06 07 08 00 00 00 00 00 00

LONG Message

The first 8 bytes match SHORT. Bytes 8 to 71 form a fixed 64-byte data area, and byte 72 is CRC-8. Classic CAN uses only the first 0 to 8 data bytes; fill the rest with 00.

Type	Bytes 0 to 7	Bytes 8 to 71	Byte 72
Extended	`ET + Channel + ExtID(4B) + Len`	64-byte data area	CRC-8
Standard	`ST + Channel + 00 00 + StdID(2B) + Len`	64-byte data area	CRC-8

Calculate CRC

def crc8(data: bytes) -> int:
    crc = 0
    for value in data:
        crc ^= value
        for _ in range(8):
            crc = ((crc << 1) ^ 0x07) & 0xFF if crc & 0x80 else (crc << 1) & 0xFF
    return crc

Calculate over the first 22 bytes for SHORT or the first 72 bytes for LONG and place the result in the final byte.

Read CAN Feedback

SPI is full duplex. Data read while sending a command may be older cached feedback or all 00. If the target reply is not immediately available, continue sending all-zero frames of the current length.

A valid reply has:

The total length required by the active SHORT/LONG mode.
A correct CRC in the final byte.
A message area beginning with 45 54 or 53 54.
Local Channel 01 or 02.
The valid CAN data length in byte 7.

One SPI read is not a fixed response to one request. Match feedback by CAN ID and device protocol.

Examples

SPI to CAN CyberGear example

Troubleshooting

Symptom	Action
Received data is all `00`	No feedback is queued; wait briefly and send another empty frame
CRC fails	Check Mode 0, MSB First, fixed length, SPI clock, and chip-select timing
CAN device does not reply	Check chip select, Channel, CAN ID, bitrate, termination, wiring, and power
Frame is sent on both CAN interfaces	Channel is `00`; select a specific channel
No response after switching to LONG	The host is still sending 23 bytes; send exactly 73 bytes per transfer