STM32F746zgtx How to bypass power from Vddusb for PA12 pin

Martini · ‎2025-08-25

hi,

The PCB with STM32F746zgtx has already been shipped to the customer.
Pin PA12 is responsible for controlling the RS485 DIR.
The documentation states that Pin PA12 is powered by the Vddusb pin.
It turns out that pin PA12 doesn't drive the high voltage (3.3V) because the Vddusb pin isn't connected.

I can't make any hardware changes. Is there any way to force this in software?

I've tried unsuccessfully:
1. Set Open Collector + Pullup,

2. Set Push-Pull,

3. Set PIN PA11 (which is also powered by Vddusb) as a PULL-UP input, hoping it will transfer to PA12.

4. Pin PB14 controls the LED (via ground). This pin also receives a high signal from the Vddusb pin, so I thought it might be possible to transfer the high signal from the LED to pin PA12, but it didn't work.

5. I tried setting PA12 as hardware control (RS485).

Do you have any ideas?

mƎALLEm · ‎2025-08-25

Hello,

Unfortunately, there is no solution. You need to connect VDDUSB to VDD.

As stated by you it's mentioned in the datasheet: DS10916 Rev 5 / Table 18. General operating conditions:

Even if PA12 is not used for USB, it needs to be power supplied.

To give better visibility on the answered topics, please click on "Accept as Solution" on the reply which solved your issue or answered your question.

View solution in original post

mƎALLEm · ‎2025-08-25

Hello,

Unfortunately, there is no solution. You need to connect VDDUSB to VDD.

As stated by you it's mentioned in the datasheet: DS10916 Rev 5 / Table 18. General operating conditions:

Even if PA12 is not used for USB, it needs to be power supplied.

To give better visibility on the answered topics, please click on "Accept as Solution" on the reply which solved your issue or answered your question.