UART stops printing when using 25 MHz external clock on NUCLEO-H753ZI-$AT3

[sameer_2k3]

Hello,

I am using a NUCLEO-STM32H753ZI-$AT3 development board.
When I configure the MCU to use the internal clock (HSI), everything works correctly and my UART debug logs are printed normally.

However, when I switch to using the external 25 MHz clock (HSE), no UART logs are printed. The code runs,but the UART prints garbage characters, for example:

<0x1c><0xe0><0xfc><0x1c><0xe

What I have already checked:

• 25 MHz external clock is selected in CubeMX (HSE ON, Crystal/Ceramic Resonator).

• PLL settings are unchanged except switching HSI → HSE.

• UART baud rate is the same (115200).

• If I go back to internal HSI, UART prints again.

External clock ioc and log

Iternal clock ioc and log

[Andrew Neil]

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@sameer_2k3 wrote:

The code runs,but the UART prints garbage characters, for example:

<0x1c><0xe0><0xfc><0x1c><0xe

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So your baud rate is wrong.

Use an oscilloscope or logic analyser to see what rate you are actually getting.

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@sameer_2k3 wrote:

• 25 MHz external clock is selected in CubeMX (HSE ON, Crystal/Ceramic Resonator).

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You mean an external Crystal - not an external clock?

Have you checked that it is actually oscillating, and actually at 25MHz?

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@sameer_2k3 wrote:

• PLL settings are unchanged except switching HSI → HSE.

---

But the HSI is 64MHz - changing to a 25MHz source means that you do need to change your clock settings...

[Tesla DeLorean]

Check input clock is 25 MHz with scope, usually derived from ST-LINK, check options/settings there.

Check HSE_VALUE define in stm32h7xx_hal_config.h matches reality. 

[Andrew Neil]

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@Tesla DeLorean wrote:

usually derived from ST-LINK, check options/settings there.

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@sameer_2k3 if it is coming from the ST-Link, then you would need 'Bypass' mode - not 'Crystal/Ceramic Resonator'

[ELABI.1]

Hi @sameer_2k3,

Are you using an external HSE?  The NUCLEO-STM32H753 board does not have an HSE clock fitted.

 I think this may be the cause of your problem.
You can refer to this article How to use STLINK-V3 MCO output on Nucleo boards as a precise clock source for STM32 to see how to select the frequency and configure the HSE input according to your selection.

Thanks.

ELABI.1

[sameer_2k3]

after selecting bypass mode still printing garbage

[sameer_2k3]

i have connected externally 25 M

[sameer_2k3]

You mean an external Crystal - not an external clock?

yes  external Crystal  .

Have you checked that it is actually oscillating, and actually at 25MHz?

yes checked and verify

[sameer_2k3]

on NUCLEO-H753ZI board (MB1364), the STLINK has a crystal oscillator having a value of 25 MHz.

 How to use STLINK-V3 MCO output on Nucleo boards as a precise clock source for STM32 they already mention 

[TDK]

Unless you have physically modified the board, you should select HSE bypass mode and change the value to 8 MHz. That is the clock signal coming in to OSC_IN on the target chip. The ST-link chip has a 25 MHz crystal but that is not relevant here.