HSE Bypass mode - Possible use of OSC_OUT as GPIO

BPart.1 · ‎2025-10-24

Hi I am looking into using STM32F446RCTx for my design, which includes 2 x USB FS 2.0 (1 x Host + 1 x Device). Hence need to provide an external HSE. I want to use an Oscillator, meaning I setup the HSE as "Bypass Clock Source" .

With this setup I planned to use the pin 6 (PH1_OSC_OUT) as a GPIO for other functionalities. But the STM32CubeMX setting does not let me do this.

I see that this pin will be (OSC_OUT) High Impedance as per RM0390 - Page 120. Am I right in say that I MUST leave this pin NC and cannot free up to use as GPIO?

Thanks.

mƎALLEm · ‎2025-11-18

Hello,

I've got a confirmation from the team: yes for STM32F446 you can use OSC_OUT and OSC32_OUT as GPIOs when HSE or LSE are in bypass mode.

The documentation will be updated accordingly (Internal ticket number 220390).

Hope that answers your question.

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mƎALLEm · ‎2025-10-24

Hello,

I think that possibility is available only in newer products. STM32H7 can do that.

According to the STM32F446 reference manual. It says not. But I can check internally. Internal ticket number 220390 not accessible by the community members.

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BPart.1 · ‎2025-10-24

Hi Thanks for checking this out.

BPart.1 · ‎2025-10-30

Hi, the answer to the original question what I should do with the OSC_OUT pin is fine for me to get on with the design for now. I am however is still waiting for the answer FROM ST on if there is a way of using this port as GPIO. Your associate mƎALLEm mentioned that he will find this out from ST.

Thanks

mƎALLEm · ‎2025-10-30

Hello @BPart.1 ,

I'm still waiting a confirmation from the internal team. If I have any feedback I will get back to you.

Thank you for you patience.

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mƎALLEm · ‎2025-11-18

Hello,

I've got a confirmation from the team: yes for STM32F446 you can use OSC_OUT and OSC32_OUT as GPIOs when HSE or LSE are in bypass mode.

The documentation will be updated accordingly (Internal ticket number 220390).

Hope that answers your question.

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.

waclawek.jan · ‎2025-11-18

> yes for STM32F446 you can use OSC_OUT and OSC32_OUT as GPIOs when HSE or LSE are in bypass mode

This is interesting. I've just tested and, yes, the 'F446 is different from the 'F427 (and thus presumably also 'F407) in this detail.

I've noticed another interesting and undocumented detail: in both the 'F446 and the 'F427, the RCC_CR.HSEBYP bit appears to be not reset by Software Reset, only by NRST and Power Reset.

JW

Andrew Neil · ‎2025-11-29