Internal pull-up/down resistors in the Rx/Tx pins for CAN

[kam8711]

Split from this thread.

i've had the same issue. Set the internal pull-up on Rx pin and pull-down on Tx pin.

[mƎALLEm]

@kam8711 : You should not enable pull-down on Tx pin.

[kam8711]

why?

[mƎALLEm]

In loopback mode, Tx and Rx are internally connected by hardware. If you enable the pull-up on Rx and the pull-down on Tx, a voltage divider will be formed on Tx/Rx path by the pull-up/pull-down resistors which should be avoided.

Please keep only the pull-up resistor on CAN_Rx pin.

[kam8711]

Thank you for reply. I didnt know that loopback mode works like that. But

in normal mode using pull-down on Tx pin is ok?

[mƎALLEm]

The question is: is there any reason to enable the pull up/down resistor on Tx pin?

[kam8711]

Many people do it so that is why im wondering….

[mƎALLEm · Accepted solution]

It's comprehensible but it should be a pull-up and not a pull down resistor. May be to handle the transients and to ensure the node are asserting recessive state if the pin is not yet initialized.

But you need to check if the transceiver has that pull-up on Tx internally:

Ex, MCP2551:

So if the transceiver has that resistor, you don't have to activate the resistor from the MCU side.

[kam8711]

Thank u for explanation