MC68HC912DG128 Datasheet, PDF (407/452 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Technical Data — MC68HC912DG128
Section 20. Appendix: CGM Practical Aspects
20.1 Contents
20.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
20.3 A Few Hints For The CGM Crystal Oscillator Application. . . . 407
20.4 Practical Aspects For The PLL Usage . . . . . . . . . . . . . . . . . . 410
20.5 Printed Circuit Board Guidelines. . . . . . . . . . . . . . . . . . . . . . . 415
20.2 Introduction
This sections provides useful and practical pieces of information
concerning the implementation of the CGM module.
20.3 A Few Hints For The CGM Crystal Oscillator Application
20.3.1 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).
MC68HC912DG128 — Rev 3.0
MOTOROLA
Appendix: CGM Practical Aspects
For More Information On This Product,
Go to: www.freescale.com
Technical Data
407