Understanding CAN external loopback mode

[ranran]

Hello,

STM32H7 contains example for  CAN external loopback mode.

In the readme there is no mention for the need of wiring a physical loopback between tx and rx pins.

Is it required in the external loopback mode ?

Thanks

[Uwe Bonnes]

Probably CAN_TX and CAN_RX need to be connected and have some Pull-Up.

[turboscrew]

I don't know about H7-devices, but in F4-devices the loopback is chip-internal. In normal loopback mode, the tx is connected to rx inside the chip, and the lines are also connected to the pins. In silent loopback mode, the connection to outside is cut off. It's only internal.

[ranran]

When I simply wire connect between them I don't get the transmitted frame,.

Is it not possible to do a physical loopback with can bus?

[Tesla DeLorean]

>>STM32H7 contains example for CAN external loopback mode.

This example?

STM32Cube_FW_H7_V1.5.0\Projects\STM32H743I-EVAL\Examples\FDCAN\FDCAN_Loopback

Please be SPECIFIC

[turboscrew]

Connecting Tx and Rx lines should work too, but in normal, not loopback mode.

[ranran]

It did not work when I tried with with eval. It also seems that theoretically , there is a problem in trying to do it:

https://electronics.stackexchange.com/questions/224022/can-bus-test-via-loopback-possible-or-not

[Tesla DeLorean]

The device has an internal loop back for testing. If you want to test externally you could presumably wire up both CAN1 and CAN2, so the latter can monitor and acknowledge traffic.

[turboscrew]

At least that should work.

[turboscrew]

There are two loopback modes in F4 CAN controllers - I suspect they are there in other chips too. The loopback-mode sends the data, but also receives it, if the filters are set that way. The quiet loopback mode doesn't send the data outside the chip, but the controller receives the sent message if the filters are set that way.