Regarding CAN bus

Question

I am using a 16-bit MCU PIC24HJ64GP504 to write a CAN based application. Basically it is communication between my board and one another node which continuously keeps on sending data to my board using CAN at 1 Mbit/s. I am configuring the ECAN module in my PIC24 to work at 1 Mbit/s. I have written the code in such a way that for the first 10 ms the ECAN module will accept all messages coming in from the other side and after that I have re-configured the ECAN module to accept only those messages with message ID 0x13.

Now here comes the issue... The other node and my board are powered up at the same instant. The other node starts transmitting messages after 40 ms or so after powerup. But I am not able to get any message from it on my board. Now if I power up my board first, give it some time to reconfigure the ECAN module with new filters and settle down and then power up the other node, then everything works perfectly.

Now the strangest part... If I have a CAN bus analyzer connected between my board and the other node and even if I power up both the nodes at the same time, everything works fine... No need to power up my board first. I have tried this with three different bus analyzers from different manufacturers and got the same results.

To me it appears that during re-configuration of the ECAN module, it takes some time to settle down. And with the introduction of the bus analyzer in the bus, this time is somehow cut short so that everything works perfectly. But I am not sure what exactly the problem might be.

Answer 1

CAN Communication requires at least two active devices on bus to have successful communication. This is because, a CAN frame is not completed unless someone acknowledges it.

When you power up your board and other node, it seems your board is not getting ready in 40msec. If it is not ready, it leaves "Other node" to be the only member on the bus and voilates above stated rule. Other node will get Tx error and after 128 erros, That other node will go in error mode and stop sending messages -- Hence you are not getting anything.

When you power up your board first, give it time - your board is ready and will ACK every message sent by other node -- Hence communication is good!

When you add CANalyzer, even if your board is not powered up, there are two active nodes on the bus -- Hence communication is good!

Answer 2

Also sounds like missing ACKs to me.

Are you seeing any error frames (get the scope to trigger off 6 consecutive dominant bits) - the Tx node might be going off the bus or even into some application-error mode if it doesn't get acknowledged enough.

You might be able to coax it back on bus by transmitting a dummy message on the bus.

I've found a Saleae Logic very useful in these circumstances (as well as a scope) - hang it off the Rx pin of your physical layer (or even wire up a standalone PHY that you can use to monitor the bus). The Saleae software will interpret the CAN and show you what's happening. Sometimes it's useful to use the scope trigger out to trigger the Logic.

Answer 3

The problem might be a missing ACK. The CAN-Analyzer might acknowledge frames and the device does not switch to error passive.

I would hold off sending until the whole bus is initialized.