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Forum Post: RE: AM335X External RTC crystal doesn't oscillate.

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The crystal they are using requires a load capacitance of 12.5pf.

The total load capacitance is CL = [(C312*C313)/(C312+C313)] + Cshunt, where Cshunt is the crystal shunt capacitance (C0) specified by the crystal manufacturer plus any mutual capacitance (Cpkg + CPCB) seen across the AM335x RTC_XTALIN and RTC_XTALOUT signals.

The crystal data sheet does not specify Cshunt, but this should be verified because this oscillator expect a crystal with Cshunt less than 1.5pf.  I'm not expecting the Cshunt of the crystal to be greater than 1.5pf.  However, if so they should select another crystal with a lower Cshunt.

If we assume Cshunt of the crystal is 0.5pf, a PCB shunt capacitance of 0.5pf, with the AM335x shunt capacitance being less than 0.2pf, the 18pf values used for capacitors C312 and C313 would give a total load capacitance of 10.2pf.  This is less than the load capacitance of 12.5pf specified by the crystal.

They should determine the actual Cshunt of the crystal then size the external load capacitors such the total load capacitance is 12.5pf.

Regards,
Paul


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