2-Channel Single Wire CAN Interface

1. Introduction

The Unbound.io single-wire CAN (SW-CAN) extension board is a Raspberry Pi compatible interface board intended to add one or two SW-CAN channels to a Raspberry Pi through SPI-connected CAN controller interfaces.

The board is designed to support the following configurations:

• One-channel configuration: one SPI interface, one SW-CAN channel.
• Two-channel configuration: one or two SPI interfaces, two independently selected SW-CAN channels.
• Flexible routing: the onboard switches and jumpers select the SPI bus, SPI chip-select, and interrupt GPIO assigned to each channel.
• Linux SocketCAN integration: when the MCP2515-compatible controller and transceiver are correctly configured, Linux exposes the channels as can0, can1, or another SocketCAN interface name selected by the kernel.

The board follows the Raspberry Pi 40-pin HAT-style electrical interface. It is not a Raspberry Pi computer by itself; a compatible Raspberry Pi, suitable power supply, enabled SPI controller, and correctly configured Device Tree Overlay are required.

1.1 Important terminology

2. Hardware connection introduction

2.1 Raspberry Pi 40-pin connector

The following table uses the BCM GPIO numbering and signal names specified by the Waveshare 2-CH CAN HAT MCP2515 interface reference. Physical pin numbers are included for wiring verification.

Standard MCP2515 signals from the Waveshare reference:

2.2 SW-CAN field connection

Connect the board SW-CAN channel to the target network according to the vehicle or network wiring specification. Verify the required signal names, signal return/ground, termination, and transceiver operating mode before applying power. Do not assume that an SW-CAN line can be wired like a two-wire high-speed CAN bus.

3. Hardware switch and jumper configuration

3.1 Configuration rules visible on the board

The board photograph contains the instruction:

For each of S2, S3, S4, and S5, set exactly one selector position to ON. Never enable two positions in the same selector because this can connect multiple signal sources together.

The ON direction is marked on the switch body. The board must be powered off while changing switches or jumpers.

3.2 Switch functions extracted from the board photograph

The photograph is not sufficiently high-resolution to guarantee the exact left-to-right text of every S4 position. Read the silkscreen on the physical board and select the position that matches the intended SPIx-CEy route.

3.3 J1 and J2 jumper headers

The supplied photograph identifies the jumper headers as follows:

J1 and J2 are channel-specific routing/configuration jumpers. They are used together with S2–S5 to select the SPI channel, interrupt routing, and chip-select routing. The supplied photograph does not provide enough information to assign an unambiguous electrical meaning to L, M, and H. Therefore:

1. Use the board schematic or the documentation supplied with the exact Unbound.io board revision to identify the meaning of L, M, and H.
2. Install one jumper cap only in the documented position for each channel.
3. Do not short two positions together unless the schematic explicitly requires it.
4. Power off the Raspberry Pi before changing J1 or J2.

3.4 Recommended initial configuration

For a Raspberry Pi using the standard Waveshare MCP2515 mapping, begin with the following logical assignment:

Then configure the physical selectors so that the board actually routes those signals. If the board is configured for alternate interrupt GPIOs or SPI1, the Device Tree Overlay values must be changed to match.

4. Software configuration

4.1 Operating-system prerequisites

This section follows the Raspberry Pi MCP2515 procedure in the Waveshare 2-CH CAN HAT reference:

https://www.waveshare.net/wiki/2-CH_CAN_HAT

Install the CAN utilities and ensure the kernel contains SocketCAN and the MCP2515 driver:

sudo apt update
sudo apt install -y can-utils

Enable SPI using raspi-config, or verify that SPI is enabled in the boot configuration:

sudo raspi-config

Select Interface Options -> SPI -> Enable, then reboot if requested.

4.2 Raspberry Pi Device Tree Overlay

For Raspberry Pi OS with Bookworm/Trixie or Ubuntu, the configuration file is normally:

/boot/firmware/config.txt

On older Raspberry Pi OS installations it may be:

/boot/config.txt

Back up the file before editing:

sudo cp /boot/firmware/config.txt /boot/firmware/config.txt.bak
sudo nano /boot/firmware/config.txt

For the standard two-channel MCP2515 mapping shown by Waveshare, append:

dtparam=spi=on
dtoverlay=mcp2515-can1,oscillator=16000000,interrupt=25
dtoverlay=mcp2515-can0,oscillator=16000000,interrupt=23
dtoverlay=spi-bcm2835-overlay

This example assumes:

• the MCP2515 oscillator is 16 MHz;
• CAN0 uses SPI0 CE0 and GPIO 23 for INT0;
• CAN1 uses SPI0 CE1 and GPIO 25 for INT1;
• the board switches and jumpers are configured to the same routing.

If the actual board oscillator is not 16 MHz, replace 16000000 with the measured or schematic-specified oscillator frequency. A wrong oscillator value causes incorrect CAN timing.

For a one-channel configuration, enable only the controller that is physically connected and routed, for example:

dtparam=spi=on
dtoverlay=mcp2515-can0,oscillator=16000000,interrupt=23
dtoverlay=spi-bcm2835-overlay

If INT0, INT1, CS0, or CS1 is changed using the board selectors, the overlay must be changed to match the selected GPIO/bus/chip-select. The overlay and hardware must never describe different connections.

Reboot after saving:

sudo reboot

Verify initialization:

dmesg | grep -Ei 'spi|mcp2515|can'
ls /sys/class/net
ip -details link show

4.3 Bring up a SocketCAN interface

Set the CAN bitrate according to the network specification. The following uses 1 Mbit/s, matching the Waveshare example:

sudo ip link set can0 down 2>/dev/null || true
sudo ip link set can0 type can bitrate 1000000
sudo ip link set can0 up

sudo ip link set can1 down 2>/dev/null || true
sudo ip link set can1 type can bitrate 1000000
sudo ip link set can1 up

Check the result:

ip -details link show can0
ip -details link show can1

4.4 Linux SocketCAN send and receive example

Install can-utils if it is not already installed:

sudo apt install -y can-utils

In terminal 1, receive on CAN0:

candump can0

In terminal 2, send an 11-bit CAN data frame on CAN1:

cansend can1 000#11.22.33.44

For a single-board bench test, connect the two SW-CAN channels only as permitted by the board and network design, and use a suitable test network. Do not connect the board directly to an unknown vehicle bus during initial software validation.

Stop the interfaces with:

sudo ip link set can0 down
sudo ip link set can1 down

5. Testing standards

The product is tested according to the LGM SW-CAN standard specified for the intended application. The applicable released LGM test specification, revision, and test limits shall be recorded in the project quality record.

The test program should cover, at minimum:

• correct Raspberry Pi 40-pin electrical connection;
• SPI communication and MCP2515 register identification;
• correct chip-select and interrupt routing for each enabled channel;
• SW-CAN transmit and receive operation at the required network bit rates;
• CAN error handling and recovery;
• power-up, reset, and repeated initialization;
• physical-layer signal integrity, polarity, and termination as required by the LGM SW-CAN standard;
• operation of one-channel and two-channel configurations;
• verification that unused selector positions do not create signal contention.

The statement “tested according to LGM SW-CAN standard” is a product test claim, not a substitute for the controlled LGM standard document. Use the current approved LGM revision and retain the test results, equipment calibration records, wiring configuration, switch settings, and software version.

References

1. Waveshare, 2-CH CAN HAT, Raspberry Pi 40-pin interface, MCP2515 overlay, and SocketCAN examples: https://www.waveshare.net/wiki/2-CH_CAN_HAT
2. Waveshare English reference: https://www.waveshare.com/wiki/2-CH_CAN_HAT
3. Linux SocketCAN documentation: https://www.kernel.org/doc/Documentation/networking/can.txt

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