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pgregson1
pgregson1
Associate III
February 24, 2019
Question

Question on procedure for calculating crystal loading capacitors...

  • February 24, 2019
  • 2 replies
  • 780 views

Is the calculation procedure in AN2867 "Oscillator design guide for STM8AF/AL/S and STM32 microcontrollers" correct? On page 12, Cl is given by

CL = (CL1 x CL2)/(CL1 + CL2) + Co

where CL1 and CL2 are loading capacitors and Co is the crystal shunt capacitance. What about the PCB and pin capacitances? AN2867 says that a suitable figure for them is 10 pF and since the series combination of them is in parallel with Co, shouldn't the equation above be

CL = (CL1 x CL2)/(CL1 + CL2) + Co + 10pF/2

Similarly, the equation for gmcrit on page 13 should include pin and PCB capacitance, should it not?

And what about the calculation for ESR on page 14?

Also, in the equations at the bottom of page 15, why is Co ignored? And is Cs the capacitance of the PCB and pins?

Please advise.

Best regards,

Peter

This topic has been closed for replies.

2 replies

Amel NASRI
Amel NASRI
ST Technical Moderator
February 25, 2019

Which product are you using?

To give better visibility on the answered topics, please click on "Best Answer" on the reply which solved your issue or answered your question.
pgregson1
pgregson1Author
Associate III
February 25, 2019

The STM32L476RGT6. But does it matter? Obviously the numbers depend on the specifics found on crystal and processor data sheets, but my question is with respect to the method. Since I use a number of different processors regularly, I need clarification on the method so as to select the correct crystals for all of them.

Best regards,

Peter

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