CAN bus is a protocol used in hundreds of millions of devices including cars, trucks, buses, construction machines, ships, tractors, and spacecraft (there is even a CAN bus orbiting Mars). The Canis Labs CANPico a carrier board in a ‘sock’ format for a Raspberry Pi Pico: simply solder on the Pico and it is connected to an advanced CAN controller and transceiver, ready for connection to CAN bus wires via a simple screw terminal. The CANPico includes an instrument header with the analog CAN H and CAN L signals and the digital RX and TX signals for use with an oscilloscope or logic analyzer. Software support is via an open source MicroPython SDK.

The open source MicroPython SDK from Canis Labs provides a sophisticated CAN API, including priority-inversion free drivers, 1μs accuracy timestamps, large buffers and an API for triggering a logic analyzer or oscilloscope. And there is support for communicating to a host device over USB (via the MIN protocol) to enable applications like bus logging or even using a CANPico as a PC CAN interface. The SDK also includes the CANHack toolkit for low-level error injection. The SDK lets Python code run on the dual-core RP2040 microcontroller of the Raspberry Pi Pico and respond in real-time to CAN traffic with times measured in microseconds rather than milliseconds, making it a great platform for hardware-in-the-loop test and emulation.

Main hardware features

• Microchip MCP2517/18FD CAN controller with 2Kbyte buffer space
• Microchip MCP2562FD CAN transceiver
• Screw terminal for direct access to CAN High/Low twisted pair wiring and a common ground reference
• Jumper for a standard 120Ω CAN bus termination resistor
• Jumper for enabling and disabling transmit input to the transceiver

Interfacing

• SPI interface at up to 18MHz
• Header pins for logic analyzer access to digital CAN signals (TX and RX pins of the transceiver)
• Header pins for oscilloscope access to the analog CAN signals (CAN High/Low, ground)
• TRIG pin for logic analyzer and oscilloscope triggering on programmable CAN events
• Direct GPIO access to CAN RX and TX pins for software error injection
• GPIO control over transceiver low power standby

The Raspberry Pi Pico is designed to be soldered on to a board using the ‘half moon’ edges, leaving header pins free. The other connections are USB (for power and connectivity to a host) and the CAN bus. The 6-pin header is intended for a logic analyzer and oscilloscope to see raw CAN signals, the yellow jumper is for enabling the termination resistor, and the blue jumper enables transmission on the bus (if this is removed then the board is restricted to only listening to the CAN bus, a useful security feature).

Software support

The software support is provided with a MicroPython SDK that has the following APIs:

• CAN API
• CANHack toolkit API
• Trigger API for triggering test instruments on CAN errors and CAN frames with a specified ID and payload
• MIN API to communicate with a host PC over a second virtual USB serial port

The CAN API consists of the following Python classes:

• The CAN class provides control and status of the CAN controller
• The CANFrame class encapsulates CAN frames, which are either created in software or by receiving them from the CAN controller hardware
• The CANError class encapsulates CAN error frames, which are created receiving them from the CAN controller hardware
• The CANID class describes a CAN ID: every CAN frame has a CAN ID, but many frames can have the same ID
• The CANIDFilter class describes how a CAN controller should identify and accept CAN frames based on their CAN ID

The MIN API provides MIN protocol support for reliable communication with a host PC. Some of the CAN API calls use a binary representation to allow items to be carried over MIN.