Confusion with selecting the right load capacitor for crystal

[sangar]

Hello all,
I am confused with selecting External load capacitor for Crystal(ABM3B-8.000MHZ-10-1-U-T )which have load capacitance of 10pF.Right now, I have used 10pF as external load capacitors in My circuit.But, Atmel says that to use the following formula to calculate external load capacitor..

CLEXT = 2(CL – CSTRAY – CSHUNT)
Where :
   CL =10pF= Crystal Load capacitance. The value of CL can be found in crystal datasheet
   CSTRAY = Parasitic Capacitance of the crystal pins ( XIN, XOUT) and PCB
   CSHUNT =3 to 7pF= Shunt capacitance of the crystal

CSTRAY = [(CXIN * CXOUT) / (CXIN + CXOUT)] + CPCB
As per SAM L21 device datasheet,  CXIN = 5.8pF, CXOUT = 3.2pF
Parasitic Capacitance (CPCB) of PCB is 0.5pF

External capacitor value after formula:

CSTRAY = [(5.9pF * 3.2pF) / (5.9pF + 3.2pF)] + 0.5pF = 2.5pF

CLEXT = 2(10pF – 2.5pF - 7pF) = 1pF

So, Should I change My external load capacitor from 10pF to 1pF?

Please suggest..


Thanks,
Sangar

[madires]

If the crystal oscillates fine with 10pF then keep the 10pF caps. For time critical stuff replace one cap with a trimmer cap for adjusting the frequency. From my experience the ATmega's oscillator runs fine with 18-22pF load caps.

[Kleinstein]

The C_shunt that is internal to the crystal might very well be already considered for the load. The only part to consider would than be C_shunt that is external to the crystal. Otherwise it would be odd to specify 10 pF load capacitance if 3-7 pF of this are already inside.