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CYeow.1
CYeow.1
Visitor II
April 12, 2021
Solved

STM32L4R5 HSE Rext External Resistor

  • April 12, 2021
  • 1 reply
  • 1418 views

Hi, I'm using STM32L4R5 and I'm planning to use the HSE, and already selected a crystal (Abracon LLC - ABMM2-8.000MHZ-E2-T https://abracon.com/Resonators/ABMM2.pdf).

Freq - 8MHz

CL1=CL2 = 24pF

gmcrit = 0.4548

Gain margin = 16.49

For external resistor Rext, it says

"If the power dissipated in the crystal is higher than the value specified by the crystal manufacturer, the external resistor RExt becomes mandatory to avoid overdriving the crystal. If the power dissipated in the selected quartz is lower than the drive level specified by the crystal manufacturer, the insertion of RExt is not recommended and its value is then 0ohm."

How do I know the power dissipation of the crystal? And is Rext necessary?

Thanks.

Oscillator design guide - https://www.st.com/resource/en/application_note/cd00221665-oscillator-design-guide-for-stm8afals-stm32-mcus-and-mpus-stmicroelectronics.pdf

This topic has been closed for replies.
Best answer by Peter BENSCH

You are referring to section 3.5.3 of that AN2867. The calculation in this section gives approx. 165 ohms as the maximum value for Rext and your current crystal to ensure stable oscillation.

Rext is in most cases not necessary and can be substituted by a 0-ohms jumper to prepare for any eventuality. But to be 100% sure, you should carry out the measurement according to section 3.5.1 (drive level).

When your question is answered, please close this topic by choosing Select as Best. This will help other users find that answer faster.

Good luck!

/Peter

1 reply

Peter BENSCH
Peter BENSCHBest answer
ST Technical Moderator
April 13, 2021

You are referring to section 3.5.3 of that AN2867. The calculation in this section gives approx. 165 ohms as the maximum value for Rext and your current crystal to ensure stable oscillation.

Rext is in most cases not necessary and can be substituted by a 0-ohms jumper to prepare for any eventuality. But to be 100% sure, you should carry out the measurement according to section 3.5.1 (drive level).

When your question is answered, please close this topic by choosing Select as Best. This will help other users find that answer faster.

Good luck!

/Peter

In order 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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