NUCLEO-H755ZIQ 8MHz HSE instead of 25MHz?

Gustavo_AR · ‎2024-09-09

I have several NUCLEO-H755ZI-Q boards, the documentation says that the clock coming out from STLINK should be 25MHz but I meassured STLK_MCO and found an 8MHz signal... on SB72 to be more precise.

The sticker on the boards say

NUCLEO-H755ZI-Q

NUH755ZIQ$AT3

I was having issues when using HSE as a clock source, because I was configuring it to 25MHz as stated in the documentation and as it defaults to in CUBEMX... The issues went away when I set it to 8MHz.

Under the microscope X1 crystal shows 25.000.

Is there something to configure on STLINK to get the 25MHz?

Could the board be fake?

SofLit · ‎2024-09-10

Hello @Gustavo_AR ,

Becarefull, the boards having STLINK V3, can generate different frequencies on STLINK MCO according to the mode selected using the STLINK firmware upgrade tool:

NUCLEO-H755ZI has STLINK V3 onboard.

By default it's HSE/3 = 8.33MHz and not 8MHz:

And you can change this according to the menu in STLINK firmware upgrade tool.

Need to update the user manual of the board (internal ticket 190627).

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.

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Pavel A. · ‎2024-09-09

This is what the documentation says on the NUCLEO-H755ZI-Q HSE input from ST-LINK: 8 MHz.

Gustavo_AR · ‎2024-09-10

Thank you @Pavel A. My mistake was assuming that the frequency of the STLINK crystal oscillator went directly to the MCU. This assumption was reinforced by the fact that when starting a project on CubeMX for this board, the frequency for HSE is set to 25MHz. However, this should be the setting for the unpopulated X2 crystal.

Regards!

SofLit · ‎2024-09-10

Hello @Gustavo_AR ,

Becarefull, the boards having STLINK V3, can generate different frequencies on STLINK MCO according to the mode selected using the STLINK firmware upgrade tool:

NUCLEO-H755ZI has STLINK V3 onboard.

By default it's HSE/3 = 8.33MHz and not 8MHz:

And you can change this according to the menu in STLINK firmware upgrade tool.

Need to update the user manual of the board (internal ticket 190627).

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.

Gustavo_AR · ‎2024-09-10

Thank you @SofLit. Mine was updated through VSCode STM32 extension. Now, by using the firmware upgrade tool I can see that MCO output is set to HSI/2 (8Mhz).

I consider that using 8.33MHz as a setting for HSE will incur in timing errors due to the decimals that are being left out. If precise timing is important I should better stick to HSE/4 or HSE/5 to have a precise time base tied to the crystal. I guess...

Thanks!!!

SofLit · ‎2024-09-10

@Gustavo_AR wrote:

I consider that using 8.33MHz as a setting for HSE will incur in timing errors due to the decimals that are being left out. If precise timing is important I should better stick to HSE/4 or HSE/5 to have a precise time base tied to the crystal. I guess...

I would say better to use HSE/5 = 5MHz instead of HSE/4 (25MHz/4 = 6.25MHz) due also to the decimals ;)

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.

Gustavo_AR · ‎2024-09-10

But 25/4=6.25 ends up there, and 25/3=8.3333333333333333333333333 infinite number of threes.

Thanks for your help!

LCE · ‎2024-09-10

If "precise timing" of anything is your goal, do not use any HSI source.

Better to live with the decimals - which are easily decimated (pun intended) with the correct PLL settings.