CAN xcvr layout questions

Thread Starter

m121212

Joined Jul 24, 2011
89
Hello,
I am unfamiliar with CAN bus and had some basic layout questions.

I have a design using a transceiver (TCAN334). I put the transceivers right next to the main connector to keep the output lines short.

1) Regarding the differential outputs (CANnH/CANnL), I am rationalizing that since the lines are so short, I a) don't need to worry about differential routing/ trace length matching, and b) don't need to worry about microstrip layout design or having a controlled impedance for the PCB. In the layout picture, the connector holes are 100mil pitch.

2) Regarding the inputs (CnRX and CnTX), these are as high speed as the outputs, but single-ended. Because the xcvrs are so close to the connector, that makes them fairly far from the MCU (~15 inches!). Do these need transmission line design, necessitating controlled board impedance/material stackup?

This thread makes it seem I don't need to worry, but I am unsure:
https://electronics.stackexchange.com/questions/228377/can-bus-layout-design

Thanks,
m121212

canxcvr.png
 

Papabravo

Joined Feb 24, 2006
13,721
Maximum CAN bitrate is 1 Mbits/sec so this is not really transmission line territory. The transceivers are not true differential drivers/receivers. They are quasi-differential WITH RESPECT TO A COMMON GROUND. Whatever the ground reference is for the transmitter, all the receivers on the network MUST be referenced to the same ground. This ground can have common mode differences between nodes at opposite ends of a long cable. Check specifications on transmit levels and receiver thresholds for dominant and recessive bits.

I would be more worried about the trace inductance of single ended logic level signals going to and from the transceiver. IMHO this is where you are going to have a problem.
 

Thread Starter

m121212

Joined Jul 24, 2011
89
Thanks.

Regarding trace inductance, what if I put Schmitt buffers mid-trace? A repeater of sorts? Or wide tracks, or both?
 

Papabravo

Joined Feb 24, 2006
13,721
Thanks.

Regarding trace inductance, what if I put Schmitt buffers mid-trace? A repeater of sorts? Or wide tracks, or both?
I'm not really a layout guy so I can't tell you what has any probability of working. In all the board I've ever designed and had built over half a century, I've never had one with a 15" trace.
 
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