Electronic – MCP2551 with STM32 MCU

Even though the device is self-powered, you still need to connect the USB's Vbus (pin 1) to the microcontroller's OTG_FS_VBUS (pin PA9).

This isn't to provide power, but to allow the microcontroller to know when to start up the USB subsystem.

One as transmitter and the other as receiver

It doesn't work that way. CAN is a peer to peer network. Any node can send a message at any time, and all the other nodes on the network receive the message.

constant 3,3V at the STM32 TX

This is clearly a firmware bug. The signal isn't getting to the CAN bus transceiver chip, let alone the CAN bus itself.

Things to be aware of:

1. The CAN lines need to be pulled together by about 60 Ω. That should be 120 Ω at each end of the bus. If the bus is really short, like a few inches on a single PC board, then a single 60 Ω resistor can be acceptable.

   On a real bus, this terminates the line to avoid reflections. However, this "termination" is still required even when the whole system is lumped. The resistance functions as a pull-together to keep the bus in the recessive state when not explicitly driven to the dominant state. This termination is not optional, even when it has nothing to do with terminating the transmission line.

2. You need at least two functioning nodes to have a working CAN bus. This is because the transmitter of a message expects to see the ACK bit asserted. With nothing but the transmitter on the bus, nothing will assert ACK, and the transmitter will think the message was corrupted and resend it. But see point 3.

3. The CAN standard attempts to prevent broken nodes from taking down the whole bus. If a node sees too many error conditions, it will shut itself down for a while. If you have only one node that never sees ACK, some of this error logic might get tripped after a while. I don't remember off the top of my head what conditions cause what level of disabling, but not having a full bus could cause the single node to disable itself, at least somewhat.