MC68HC12D60 Datasheet, PDF (387/432 Pages) Motorola, Inc – Advance Information - Rev 4.0

Freescale Semiconductor, Inc.
Advance Information — 68HC(9)12D60
Section 21. Appendix: CGM Practical Aspects
21.1 Contents
21.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
21.3 A Few Hints For The CGM Crystal Oscillator Application. . . . 387
21.4 Practical Aspects For The PLL Usage . . . . . . . . . . . . . . . . . .390
21.5 Printed Circuit Board Guidelines. . . . . . . . . . . . . . . . . . . . . . .395
21.2 Introduction
This sections provides useful and practical pieces of information
concerning the implementation of the CGM module.
21.3 A Few Hints For The CGM Crystal Oscillator Application
21.3.1 What Loading Capacitors To Choose?
First, from small-signal analysis, it is known that relatively large values
for C1 and C2 have a positive impact on the phase margin. However, the
higher loading they represent decreases the loop gain. Alternatively,
small values for these capacitors will lead to higher open loop gain, but
as the frequency of oscillation approaches the parallel resonance, the
phase margin, and consequently the ability to start-up correctly, will
decrease. From this it is clear that relatively large capacitor values
(>33pF), are reserved for low frequency crystals in the MHz range.
NOTE:
Using the recommended loading capacitor CL value from the crystal
manufacturer is a good starting point. Taking into account unavoidable
strays, this equates to about (CL-2pF).
68HC(9)12D60 — Rev 4.0
MOTOROLA
Appendix: CGM Practical Aspects
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Advance Information
387