USB to CAN Adapter

USB to CAN converter, with USB C port and CAN FD SIC transciever supporting speeds up to 8mbps. Two JST-GH connectors are used for the CAN bus. The converter does have any termination resistors, if required they need to be added externally.

Small board size: L: 22.5mm, W: 13mm, T: 7mm.

The CAN bus will function correctly without termination resistors if the cable length is under 50cm and all devices on the network use CAN SIC transcievers. This mode of operation also saves power and does not need careful consideration of the cable impedance.

For longer cable runs, a termination resistor is required to prevent signal reflections in the cable from causing problems. The value of the termination resistor needs to be as close as possible to the characteristic impedance of the cable. Do not just put a 120 ohm resistor as the termination resistor without knowing the characteristic impedance of the cable, it will not work.

The choice of cable is also very important, as it needs to have a constant characteristic impedance, and preferably be a differential pair. Some sources of cable you can use:

• CAN Bus cable, which provides the best signal integrity, is a twisted pair, has a characteristic impendance of 120 ohm, but is very expensive
• A twisted pair from an Ethernet cable, which provides good signal integrity, has a characteristic impedance of 100 ohm, and is cheap and readily available.
• Any 2 core cable, such as a section cut from a ribbon cable, which does not have great signal integrity due to not being a twisted pair, has a varying characteristic impedance you need to measure yourself, and is probably the cheapest type of cable available which will work.
• Do NOT use 2 separate cables to carry the CANH and CANL signals, it will not have a constant characteristic impedance and will not work. 

The performance the CAN Bus has been tested using a 30m long jumper wire ribbon cable. The characteristic impedance of the cable was measured to be 200 ohms using trial and error method with an oscilloscope. A 200 ohm termination resistor was installed at both ends. The signal integrity was measured with the oscilloscope and was excellent, and there were no issues communicating at 8mbit/s.

Note about DFU bootloader and USB C:

When the DFU bootloader is used, the onboard USB PD controller pins get reconfigured by the bootloader, causing USB C functionality to no longer work. This means that if you try to use the bootloader to upload firmware while using a USB C to C cable, it will not work. A workaround is to use a USB A to C cable. Only bootloader mode is affected by this issue, USB C to C cables can be used in normal mode without issues.