Hello
I'm using Abracon 25MHz( ABM11W) crystal for HSE clock on STM32H750.
CL1 and CL2 are set to 12pF after some calculations. And it works on the prototype.
Question is regarding the resistor do reduce/control driving level to this crystal. I have been looking into ST application note "AN2867 Pierce oscillator design" and chapter 3.5.3 Calculating the external resistor.
According to DS for the crystal the maximum driving level is 10uW typ and max 100uW.
If I calculate the Rext I get 531 ohms to start. ( Rext=1/(2*PI()*25MHz*12pF)
Then the app note recommends a potentiometer and turn from 531 ( I guess ) "until an acceptable output and crystal drive level are obtained".
Questions.
1) How do I define a good crystal drive level when I measure with oscilloscope ?
2) What is the drive level from STM32H750 to drive a crystal in uW? it this number is lower than 100uW the resistor can be taken away.
3) In this case, do you have any Resistor value to recommend? I get a feeling of inventing an old wheel when I'm spending time to figure out the resistor value for Rext. I can't be that complex or difficult.
Thank you for your help!
https://abracon.com/Resonators/ABM11W.pdf
