STM32H7B3LIH6Q: Crystal Selection Impact on CubeMX Clock Configuration

[KMew]

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.

 

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.

 

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?

[gbm]

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]

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

[AScha.3]

or just take an oscillator - read here:

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

[KMew]

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?

 

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?

[KMew]

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

[AScha.3]

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. )

[KMew]

Ahhh, I understand!

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