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STM32H7B3LIH6Q: Crystal Selection Impact on CubeMX Clock Configuration

KMew
Senior III

Hello,

I am designing a custom board using the STM32H7B3LIH6Q, based on the STM32H7B3I-EVAL evaluation board. The crystal used on the evaluation board is 24 MHz, as seen from the schematic offered.

0693W00000Y8gYWQAZ.png 

Now, I am building my own board and noticed that the part number used for the oscillator is not available. However, there the datasheet says this one can be a range (4 - 50 MHz). See datasheet snippet below.

0693W00000Y8gYqQAJ.png 

My question is two fold:

  1. When they say "guaranteed by design" in the second note, does that mean that, if I use a crystal between 4-50 MHz, I can achieve the full 280 MHz clock frequency used by this MCU?
  2. Does choosing a crystal with a different frequency cause changes in the the CubeMX clock configuration?

1 ACCEPTED SOLUTION

Accepted Solutions
AScha.3
Chief II

or just take an oscillator - read here:

https://community.st.com/s/question/0D53W0000249fD6SAI/how-to-calculate-the-oscillator-limiting-resistor

If you feel a post has answered your question, please click "Accept as Solution".

View solution in original post

7 REPLIES 7
gbm
Lead III

Just look at the Clock Configuration tab in CubeMX - this should answer your questions. If in doubt, read the Ref Manual section on RCC.

Imen.D
ST Employee

Hello @KMew​,

When selecting your crystal, be sure to follow the recommendations in the AN2867:

Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs - Application note

Imen

When your question is answered, please close this topic by clicking "Accept as Solution".
Thanks
Imen
AScha.3
Chief II

or just take an oscillator - read here:

https://community.st.com/s/question/0D53W0000249fD6SAI/how-to-calculate-the-oscillator-limiting-resistor

If you feel a post has answered your question, please click "Accept as Solution".

Hello AScha.3,

Thank you for the reply!

I read through this thread and it was very helpful! An oscillator does seem simpler for the application. I have two follow-up questions about them, just to make sure I understand:

1) What exactly is the "tristate" of the oscillator for the example you mentioned in your reply?

0693W00000Y8mQdQAJ.png 

2) I noticed you didn't use the 0.01 - 0.1 uF capacitor in your example, but the datasheet does mention one in Figure 2. I also noticed you added a 51 ohm resistor between output and the MCU. Is that necessary?

Added the picture of your example schematic, for ease of reading.

0693W00000Y8mRHQAZ.png

AScha.3
Chief II
  1. just to switch off the clock (maybe you want go low power and use cpu internal slow clock)
  2. is there !!! needs to be close to osc , i use 10uF ceramic 0805 ; (just on other part of drawing)
  3. 51 or 75 r is to prevent ringing or reflections of hi-speed signals (these osc. can be very fast and create strong ringing even on short tracks; osc needs to be close to cpu.

(i had a sometimes (!!) not working cpu, 48MHz osc. only 10mm distance; so strong ringing on this short line !

cut it, put 51 r in , and cpu working always perfect. )

If you feel a post has answered your question, please click "Accept as Solution".

Ahhh, I understand!

Thank you for explaining it Ascha! You have been most helpful 🙂