Micrel, Inc.

SM840021

March 2010

9

M9999-031810-B

Total capacitance seen by crystal = CL =

1

1

+

1

(Ct1 + Cb1 + Cd1)

(Ct2 + Cb2 + Cd2)

Example:

CL = 18pF, Cb = 2pF, Cd = 4pF

Trim Cap = Ct = 2 (18pF) - (2pF +4pF) = 30pF

The SM840021 has been characterized with 25MHz,

18pF parallel resonant crystal. The trim capacitors Ct1

and Ct2 were optimized to minimize the ppm error.

To minimize the board capacitance, a short trace from

pin to crystal footprint without vias is desirable. It is

preferable to have ground shielding or distance between

the crystal traces and noisy signals on the board.

Board Layout