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.

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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.