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  • Author Author: Jan Cumps
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Arduino MKR CAN Shield Review - part 1: Log BUS Traffic

Jan Cumps

I'm trying out CAN communication with the Arduino MKR CAN Shield.

 

I have a CAN bus test setup in the lab, with two DIY CAN drivers.

If all is OK, I should be able to write firmware and plug the Arduino CAN shield onto that BUS.

It can then snoop the data - and in a second step, send packets.

 

image

 

Introducing Arduino's MKR CAN Shield

 

The Arduino MKR series is rather interesting. A number of small-sized controller boards and a set of useful shields that match that size.

The CAN shield that I review here uses a popular CAN controller, the Microchip MCP2515Microchip MCP2515, and as CAN driver the NXP TJA1049T/3NXP TJA1049T/3.

image

image source (note: the actual screw terminals on the board I have are different): arduino.cc

 

Highlights:

  • bus termination resistors that can be switched in or out.
  • an on-board buck converter that allows you to power the Shield and an Arduino MKR 1xxx controller from external 7 to 24 V dc.
  • Allows for interrupt driven message receive and hardware filtering for relevant data.
  • A choice of supporting libraries and example sketches.

 

Wiring the CAN Shield in a CAN Bus

 

The CAN bus is an easy bus to add devices to. In my case,

I have an existing 3V3 CAN bus existing of

  • two CAN breakout boards using a TI CAN driver and a Hercules automotive microcontroller for the logic.
    They are currently programmed to send random data upon a button push and listen to specific packets.
  • a Microchip CAN Bus AnalyzerMicrochip CAN Bus Analyzer.
    Versatile, can listen, send, show protocol errors and log.

 

My two breakout boards are set up as termination points of the bus. They have termination resistors to properly close the bus at the two sides.

The analyser is plugged somewhere in between these two. The analyser's termination resistors are disabled, because it's not at the end of the bus.

Similarly, when wiring in the CAN shield somewhere on the bus, the switch that configures the on-board resistors has to be switched off.

image

So, the 4 things to do when adding the MKR CAN Shield to your bus are:

  • connect HI to Bus HI
  • connect LO to Bus LO
  • connect GND to Bus GND
  • set the switch to the right.

 

First Test: Snoop CAN messages

 

We can get started straight away. That's the magic of the Arduino world. There's always an example that gets you running.

In the case of shields with a Microchip MCP2515, there are a few options. Several good libraries have been published.

I made a choise based on the example I need more than on what library I prefer. I went for Sandeep Misty's CAN lib. Its CANReceiver example has all I need.

You install the lib by searching for MCP2515 in the Arduino library manager.

image

Once installed, open the CANReciever example.

Load it to your Arduino MKR (I'm assuming you have an Arduino MKR controller at this point image ) , run it and open the serial monitor

All traffic on the bus will apear in the serial monitor.

 

image

What have you achieved?

  • Added the Arduino MKR CAN shield as a node to a CAN bus
  • Have a working device that can see what's happening on the CAN bus. Pushing data on the bus is just a little step away.

 

I think that's exciting.

 

 

Road Test Blog
part 1: First trials
part 2: Inject CAN Messages
part 3: Analyzer as Test Tool
part 4: Analyze the Physical layer of CAN Bus
Related Blog
part 1: tryout
part 2: Communication between 2 Devices
part 3a: Design a Bus Driver PCB
part 3b: Design a Bus Driver PCB - Schematics and Custom Components
part 3c: Design a Bus Driver PCB - Layout
part 3d: Design a Bus Driver PCB - Test
Arduino MKR CAN Shield Review - part 1: Log BUS Traffic
Arduino MKR CAN Shield Review - part 2: Interrupt and Filter

  • I've been testing Sandeep Mistry's CAN library further. It supports interrupts with CAN receive, and hardware filtering.

    The interrupt functionality works nicely. You can make the Arduino sleep, and the CAN device will wake it up when receiving data..

     

    I have difficulties with the filter though. The CAN controller (Microchip MCP2515) allows you to set a filter.

    When you do that, the microcontroller is only contacted when there's a message that it is interested in.

    On a CAN bus, where there can be a lot of traffic, this is useful. You don't have to triage through noise to find the data that's intended for you.

    But (further investigation needed) the library does not work as I expect it. It sets the filter registers of the CAN chip but the scope of the filter doesn't seem to match the API.

    I am trying now on lower level, setting the registers in my own code, and that gives the expected results.

     

    If it turns out to be a bug in the library, I'll fork it on github, try to fix it and contribute back to the original lib as a pull request.

    If it is my understanding of the lib that's wrong (a more likely scenario), I'll post back on it here.

  • The example here is useful to stream everything published on a bus.

    When you're implementing a real can device, it's worth checking out the library API for the filter function.

    This functionality tells the CAN device to only react to agreed messages. Hardware is filtering messages for you. It takes care that the microcontroller never has to react on (or wake up on) non-relevant packages.