CAN_RX pin state

[Jagadish]

Hello All,

I have a question related to CAN hardware in microcontroller MPC5646C.

For some specific reasons we have in our hardware a CAN controller which is started and CAN messages being transmitted. The CAN_TX and CAN_RX lines are not connected to any transceiver and are left open. In this situation, when we transmit we see that there is a bus-off occurring although we have configured CAN_RX pin as CAN specific receive functional pin.

Is this bus-off expected if the CAN lines are left open but correctly configured in software?

Regards,

Jagadish

[Jagadish]

Hello Petr,

Thanks for replying.

However what we have observed is that the bus-off does not happen always. It happens only sometimes although we transmit always. So, what we observe is that the bus-off behaviour occurs sometimes.

One thing what we plan to do is to enable the weak pull-down available in the MPC5646C microcontroller for CAN_RX. Atleast with this we do not see bus-off.

Regards,

Jagadish

[PetrS]

Hi,

Yes, this is expected behavior. If the FlexCAN sees the RX pin high, it can synchronizes to the bus , thus it is able to transmit a message. However, because of unconnected TX/RX pins, the FlexCAN does not receive what it is transmitted and so a bit error occurs. An error frame is transmitted, again with a bit error. This is repeated until bus error happen.  

BR, Petr