NUCLEO-H755ZIQ 8MHz HSE instead of 25MHz?

[Gustavo_AR]

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?

[Pavel A.]

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

 

[Gustavo_AR]

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!

 

[mƎALLEm]

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).

[mƎALLEm]

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

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FYI: UM2408 / 7.9.1 OSC clock supply section has been updated: 

"MCO from ST-LINK (default): The MCO output of ST-LINK is used as an input clock.
By default, it is fixed at 8 MHz and connected to the PF0/PH0-OSC_IN of the
STM32H7 series microcontroller. The frequency might be changed during an ST-LINK
firmware upgrade (refer to RN0093 for details)."

 

[Gustavo_AR]

Thank you @mƎALLEm. 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!!!

 

[mƎALLEm]

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@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...

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

[Gustavo_AR]

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

Thanks for your help!

[LCE]

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.