How to include OSC_IN input capacitance into oscillator Load capacitance (CL) calculation ?

[CBeguet]

Hi support team,

On STM32F4 datasheet, the Cin capacitance for OSC_IN is specified. How is measured this capacitance? I supposed it's refered to the ground. Is it?

Has the OSC_OUT pin the same capacitance (not seen in the datasheet)?

In AN2867, Cs (stay capacitor) is described as sum of PCB capacitor and pin capacitors. How shoud we consider the Cs calculation regarding to the Cin characteristic in datasheet?

If Cin is refered to ground, the Cin and Cout are respectively in parallele with CL1 and CL2. So those values can not be included in Cs calculation.

What OSC_IN and OSC_OUT capacitor should be included in Cs?

How Cin value is involved into CL calculation.

Thanks for your help

BR.

[Bouraoui Chemli]

Hello @CBeguet​ 

Cin is referred to the ground and its value is provided by design as mentioned in the datasheet.

Please refer to AN2867 for CL calculation. You can rapidly calculate CL, CL1= CL2 to simplify the calculation. The values of Cin and Cout can be neglected since they have small influence on CL calculation.

For more frequency precision you have to measure the oscillator frequency output and change CL1 and CL2 capacitors to adjust the frequency.  

Best regards,

Bouraoui