# 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_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:

```text
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:

```text
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

```python
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](https://gitcode.com/sanpo/robot/blob/unified-main/products/spine/v6/demo/can/spi2can_cybergear_demo_v6.py)

## 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 |