Microchip Technology MEC172 Series Layout Manual page 5

1.2 32.768kHz Crystal Oscillator
The following formula may be used to calculate a parallel resonant crystal's external load capacitors:

CL = ((CX1 x CX2) / (CX1 + CX2)) + Cstray

where:

CL = the crystal load capacitance
Cstray = the stray capacitance in the oscillator circuit, which will normally be in the 2pF to 5pF range.
Assuming that CX1=CX2 then the equation becomes:

CL = ((CX1 x CX1) / (2 x CX1)) + Cstray
CL = (CX1 / 2) + Cstray

Rearranging the equation, we can find the external load capacitor value:

CX1 = 2(CL - Cstray)
The formula parameters:
CL is the cystals load cap, CX1 is equivalent to C12 and Cx2 is equivalent to C13 in our doc are the cap from pin to
ground
The Cstray = trace Length cap (board layout) + pin cap (package) + pads parasitic cap (pad library)
1.2.1 SINGLE ENDED CLOCKING
An External Crystal Oscillator can be used with MEC172x for sourcing the 32kHz clock domain. For dual ended config-
uration, XTAL is connected between XTAL1 and XTAL2.
For Single ended XTAL configuration, external clock should be connected to XTAL2 pin and XTAL1 pin should be
grounded. If the 32KHz source will never be the crystal oscillator, then the XTAL1 and XTAL2 pins should be grounded.
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