Can I use CAN and USB pin only if USB plugged in?

[KYou.1]

Hello,

I am using STM32F103 for my project and it requires CAN most of the time however if a USB connector is plugged in, it should behave like a virtual COM port.

CAN_R and CAN_T pins are connected to PA11 and PA12

together with USB_DP and USB_DM pints.

Below image is the circuit I used to disable CAN transceiver when USB 5V exist.

MCU_USBV comes from the USB 5V and USB_DP is pulled up.

This circuit didn't work for me.

What need to be changed?

[Amel NASRI]

Hi @KYou.1​ ,

You find the answer to your question and the required explanation in this knowledge article: https://community.st.com/s/article/FAQ-Concurrent-use-of-USB-and-CAN-with-STM32F103.

In the article page, please answer the question "Was this article helpful?"

-Amel

[waclawek.jan]

Hi @Amel NASRI​  ,

as CAN and USB share pins, it does not appear that the problem is in trying to use both modules simultaneously.

I believe the main problem is in how the CAN transceiver is "disabled" - floating its GND pin is not a proper way, as the chip gets parasitically supplied from its other pins through the protection diodes.

A multiplexer or a three-stating gate (e.g. 74HC125) or a transmission-gate (e.g. 74HC4016) should be used to isolate the CAN transceiver's D and R from the USB DP/DM terminals.

JW