# SANPO SPI to FDCAN

SANPO SPI to FDCAN lets a Jetson, Raspberry Pi, or another host controller send and receive CAN FD standard and extended frames. CAN FD requires SPI LONG mode with exactly 73 bytes per transfer.

## Switch to LONG Before Use

Factory default is SPI SHORT, which cannot carry CAN FD. Send over USB serial as text:

```text
AT+SPIMODE=LONG
```

Use a `CRLF` line ending. After receiving `OK`, wait at least 10 ms, then start 73-byte SPI transfers. The setting is saved across power cycles. `AT+CFGERASE` restores SHORT.

Also query CAN FD bitrates:

```text
AT+SETFDCAN?
```

Factory defaults are 1 Mbps arbitration and 5 Mbps data, with BRS always enabled. The data phase can be 2, 3, 4, or 5 Mbps. See [Communication Settings](baudrate_config).

## SPI Settings

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

## Select a CAN FD 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 FD interfaces on the current MCU |

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

## Data Length Code

| Code | Bytes | Code | Bytes |
| --- | ---: | --- | ---: |
| `00` | 0 | `08` | 8 |
| `01` | 1 | `09` | 12 |
| `02` | 2 | `0A` | 16 |
| `03` | 3 | `0B` | 20 |
| `04` | 4 | `0C` | 24 |
| `05` | 5 | `0D` | 32 |
| `06` | 6 | `0E` | 48 |
| `07` | 7 | `0F` | 64 |

## 73-Byte Message

### Extended Frame

| Byte position | Content |
| --- | --- |
| 0 to 1 | Fixed `45 46` (`EF`) |
| 2 | Channel |
| 3 to 6 | 29-bit CAN ID, most significant byte first |
| 7 | Data length code `00` to `0F` |
| 8 to 71 | Fixed 64-byte data area; fill unused bytes with `00` |
| 72 | CRC-8 over the first 72 bytes |

### Standard Frame

| Byte position | Content |
| --- | --- |
| 0 to 1 | Fixed `53 46` (`SF`) |
| 2 | Channel |
| 3 to 4 | Fixed `00 00` |
| 5 to 6 | 11-bit CAN ID, most significant byte first |
| 7 | Data length code `00` to `0F` |
| 8 to 71 | Fixed 64-byte data area; fill unused bytes with `00` |
| 72 | CRC-8 over the first 72 bytes |

For Channel 1, extended ID `0x1F100101`, and 16 data bytes, the first 24 bytes are:

```text
45 46 01 1F 10 01 01 0A
01 01 01 01 42 00 00 00 01 01 64 00 78 56 34 12
```

Append 48 bytes of `00`, then append CRC-8 over the first 72 bytes. The CRC algorithm is shown in [SANPO SPI to CAN](spi_can).

## Read CAN FD Feedback

SPI is full duplex. Data read while sending a command may be an older reply or all zero. Continue sending 73-byte all-zero polling frames to read later feedback.

For each nonzero frame, check:

1. Byte 73 matches CRC-8 over the first 72 bytes.
2. The header is `SF` or `EF`.
3. Channel is local `1/2`.
4. The length code is valid.
5. CAN ID and data match the target device protocol.

## Troubleshooting

| Symptom | Action |
| --- | --- |
| Received data is all `00` | No feedback is queued; wait briefly and send another 73-byte empty frame |
| No response in 23-byte mode | CAN FD requires LONG; switch and send exactly 73 bytes each time |
| CRC fails | Check Mode 0, MSB First, fixed length, SPI clock, and chip-select timing |
| CAN FD device does not reply | Check arbitration bitrate, data bitrate, BRS, Channel, ID, termination, wiring, and power |
| Length code is valid but data is short | Always provide the complete 64-byte data area and pad unused bytes with `00` |