SN65HVD230: Layout Guideline

1. The CANH and CANL for differential signals connect from IC(SN65HVD230) to connector

a. What is the maximum total trace length on PCB?

- PCB traces inherently introduce parasitics so it is generally best to keep them short. Compared to a wiring harness, these may not be too significant for CAN signals. The more noticeable impact of CAN traces is possible emissions to other nearby traces. Higher frequency signaling may couple to/from CAN lines from/to other signal or power traces that run nearby. It is good practice to keep CAN lines away from other traces and pour ground planes nearby to dampen EMI.

b. What is the maximum trace length mismatching for CANH and CANL?

- It is best to have the trace lengths of both CAN lines to be as identical as possible. Any mis-match in length or parasitics may increase EMI emissions. Many routing software tools have a function that will help you match trace lengths for differential signals.

c. Will termination's R,C be close to IC(SN65HVD230) or connector?

- It is best to place termination close to the CAN IC. The distance between termination and the IC can be viewed as an unterminated transmission line and thus should be reduced if possible. Since the length of this segment is very short compared to the bus, this does not impact performance greatly. As an example, it would be better to include split-termination further from the IC than simple termination close-by.

d. What is the maximum trace length from IC(SN65HVD230) to termination?

e. What is the maximum trace length from termination to connector?

- There is not a particular number you should be shooting for here. Following general practices of keeping trace lengths short, especially for signal traces, will help you achieve best results. See comments for 1c regarding preferred termination placement. Note that Transient/ESD protection devices should be places near the connector. This prevents these events from penetrating deeper on the board and disturbing other devices.

2. The TXD and RXD for signals connect from MCU to IC(SN65HVD230)

a. What is the maximum total trace length from MCU to TI transceiver on PCB?

- Because these lines will also be carrying high-frequency signal data, follow comments in 1a. Isolation from other traces will be less critical because the current through these traces will be much less than CAN traces.

b. Is there a need to match the trace length between TXD and RXD? If yes, what is the maximum trace length mismatching for TXD and RXD?

- Because these are not differential signals, you don’t need to match the length here. Even though RXD will follow TXD in most cases, there will be a delay (t_loop) from the transceiver that will be more significant than is introduced from trace lengths in most cases.

To conclude; do your best to keep trace lengths short within reason, match the trace lengths of differential signals (CANH and CANL), and place ESD/Transient protection near the connector. More layout guidelines concerning decoupling capacitors, current-limiting series resistances, and external pull-up and pull-down biasing can be found in section 13 of the SN65HVD230 datasheet. 

Let me know if you have any more questions regarding CAN layout or if you would like clarification on any above comments.

Regards,

Eric

Dear Eric,

Thank you for your reply.

Because we have 3 MCUs that need to be connected by CAN bus, we need to know the following information.

a. We know that CAN lines length should be as short as possible. but it is difficult for us to put 3 MCUs very close. So we want to know the CAN lines length of the maximum on PCB trace routing?

b. We know it's best not to have a mismatch. Maybe we need the serpentining line for length matching. But it will cause impedance mismatch we can't use to much. For example. There is definitions of 20 mils mismatch for USB data pairs on Intel's PDG. So we need to know the CAN lines mismatch length of the maximum?

c. For MCU to TI transceiver . Because we have 3 MCUs to connected. So we can't close every MCUs and TI transceiver.  We still to know the TXD/RXD line length of the maximum?

 Thank you

Kevin