STM32H743XIH6 Crystal Frequency

[chand_p]

Hi ST Support Team,

I am Using STM32H743XIH6 in one of the project.

Having Query related to crystal frequency.

In datasheet frequency characteristics is provided for oscillator input.

In my Design and STM32H743XIH6 Evaluation Board crystal is implemented.

My Design contains 48Mhz Crystal while EVM contains 25Mhz crystal.

Kindly go through the attached schematic capture of developed board waveform capture at PH0-OSC_IN pin.

Getting same waveform as per Evaluation Kit (Find Attached EVM waveform)

Hope waveform we observed will specify the MCU clock requirement in terms of rise time, fall time, VIH & VIL level.

want to make sure that there is no issue in MCU Clock using external 48 Mhz crystal implementation.
in 25 Mhz & 48 Mhz crystal maximum amplitude is going at 1.8V and MCU operating at 3.3V level.
Hoping that the clock at OSC_PIN is satisfying MCU operating level.

Thanks,

[LCE]

Check the datasheet, for H733 its is:

6.3.9  External clock source characteristics
High-speed external user clock generated from an external source

fHSE_ext User external clock source frequency max 50 MHz

[chand_p]

Hi LCE,

After Referring the H733, 6.3.9.

Found, maximum Low Level of Clock need to be 0.3*VDD (= 0.7*3.3 = 0.99), in my case & in EVM it is ~1.2V
For Minimum High Level of clock need to be 0.7*VDD (= 0.7 *3.3 = 2.31), in my case & in EVM it is ~1.8V.

So considering datasheet parameter for both board not maintaining required level, correct?

Can you help me to get resolution on above concern?

Thanks..

[TDK]

Max external crystal frequency is 48 MHz, which yours meets. You should have load capacitors loaded.

 

The waveform looks fine.

 

See here for crystal selection design guide:

https://www.st.com/resource/en/application_note/an2867-oscillator-design-guide-for-stm8afals-stm32-mcus-and-mpus-stmicroelectronics.pdf

 

[chand_p]

Hi TDK,

Crystal having 8 pF load capacitance, added 8 pF capacitor at both pins and getting same result as without load capacitance.

Do need to check with other value of load capacitance?

 

Attaching snap for your reference.

Thanks..

[TDK]

I imagine the capacitance of your scope is distorting things as well.

> Crystal having 8 pF load capacitance, added 8 pF capacitor at both pins and getting same result as without load capacitance.

Just because it works doesn't mean it will continue to do so. Follow the recommendations to have the best chance of success. A little surprised it's working at all without any load caps.

 

Don't mix up specifications for the external clock input with specifications for the external crystal.

[chand_p]

Hi
Find attached snap, ignore previous one..

Thanks,

[chand_p]

Hi TDK,

Referring to attached snap considering STM32 10pF load capacitance.

 

Thanks,

[chand_p]

Hi TDK,

As per attached snap STM32 Datasheet suggesting to use Oscillator.

As per above table there is no specific requirement related to Vil, Vih level of crystal / oscillator output clock.

Can we go ahead with 48 Mhz Crystal with previous shared waveform results?

we implemented crystal in our design based on EVM 25 Mhz crystal.
If it must to have use an oscillator than what need to do with PH1-OSCOUT pin of STM32?
As in oscillator mainly providing clock output on single pin.

Kindly confirm which external source need to implement..
Thanks..
 

[TDK]

> Kindly confirm which external source need to implement..

It's up to you. You can use a crystal, or you can use an external clock source.

The 48 MHz crystal you chose will work with the appropriate load caps.

I strongly suspect that the "10 pF" value in the datasheet is misleading. Here is a post by ST to suggest the same:

https://community.st.com/t5/stm32-mcus-products/calculate-external-capacitors-cl1-and-cl2-for-crystal-oscillator/m-p/621261/highlight/true#M230537

Even if it was 10 pF, you should still have load caps of about 6 pF on there. Might want to re-check the math suggested in the guide I linked above.

> If it must to have use an oscillator than what need to do with PH1-OSCOUT pin of STM32?

If you use an external clock source, PH1 doesn't need to be connected to anything.