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SN65HVD232: PCB layout

Part Number: SN65HVD232

Dear sir,

Our customer is using SN65HVD232, and they want to ask some questions about PCB layout.

Is there any length limitation for differential signal? (CANH/CANL). What is the maximum length on PCB layout?

And what is the minimum distance of pair-to-pair signals? What is the minimum distance of pair-to-other signals?

Thank you very much.

  • CK,

    In general, the maximum length for a CAN bus is 40m at 1Mbps. With faster speeds this length will decrease. On a PCB, unless it is a very very large PCB, there won't be limitations, the designer will just have to keep in mind the impedance matching for the CAN bus, and make sure to route the CANH and CANL signals using the differential routing tool in the PCB layout software. This will help ensure they are the same length as well, to minimize any timing differences.

    I'm not sure I understand your second question. What is meant by pair-to-pair and pair-to-other signals?

    Regards,

  • Dear Eric,

    Thanks very much for your support.

    1. I know that the length of the CANH and CANL should be the same as possible. But is there any spec of the length difference between CANH and CANL?? Customers want to know this maximum value.

    2. pair-to-pair means that if I have two differential pairs or more, what is the minimum distance can be accepted between these two differential pairs? And pair-to-other means that what is the minimum distance can be accepted between this differential pair and the other high speed signal in the same layer?

  • CK,

    1. There isn't a spec for the length difference between CANH and CANL traces. Generally with PCB, you can assume a delay/speed of 180 picoseconds per inch of trace on the top layer. If the CAN data rate is 1Mbps, that means one bit width is 1 microsecond, which is 1 million picoseconds. So even with one full inch of difference between CANH and CANL, one of the signals will be delayed ~180 picoseconds, which is ~1/50 a bit width at 1Mbps. Ideally, you want your CAN bus traces to be short, symmetrical, and equal in length, but if that is not possible, just know that with slow data rates like CAN, you can afford some non-optimal CAN bus trace layout.
    2. I'm not sure the exact spec for this, but the cross-talk is caused by the magnetic field generated by the switching current in these high-speed signals. However, since CAN is differential, the equal and opposing waveforms will cancel each other out, mitigating a lot of the EM noise they would create otherwise. My advice is to leave enough distance between signal traces t fill in the space with a copper pour that is tied to the ground net. Also, placing high-frequency filter components (small value ceramic capacitors, common-mode choke) on the bus to minimize the amount of noise propagating from the bus. 

    Please let me know if you have any questions.

    Regards,