Zilog Z16C35 - Manual (Page 301)

15-1
1
A PPLICATION N OTE
O
N
-C
HIP
O
SCILLATOR
D
ESIGN
14
esign and build reliable, cost-effective, on-chip oscillator circuits that are trouble free.
PUTTING OSCILLATOR THEORY INTO A PRACTICAL DESIGN MAKES FOR A
MORE DEPENDABLE CHIP.
D
INTRODUCTION
This Application Note (App Note) is written for designers
using Zilog Integrated Circuits with on-chip oscillators;
circuits in which the amplifier portion of a feedback
oscillator is contained on the IC. This App Note covers
common theory of oscillators, and requirements of the
circuitry (both internal and external to the IC) which comes
from the theory for crystal and ceramic resonator based
circuits.
Purpose and Benefits
The purposes and benefits of this App Note include:
1. Providing designers with greater understanding of how
oscillators work and how to design them to avoid
problems.
2. To eliminate field failures and other complications
resulting from an unawareness of critical on-chip
oscillator design constraints and requirements.
Problem Background
Inadequate understanding of the theory and practice of
oscillator circuit design, especially concerning oscillator
startup, has resulted in an unreliable design and
subsequent field problems (See on page 10 for reference
materials and acknowledgments).
OSCILLATOR THEORY OF OPERATION
The circuit under discussion is called the Pierce Oscillator
(Figures 1, 2). The configuration used is in all Zilog on-chip
oscillators. Advantages of this circuit are low power
consumption, low cost, large output signal, low power level
in the crystal, stability with respect to V
CC
and temperature,
and low impedances (not disturbed by stray effects). One
drawback is the need for high gain in the amplifier to
compensate for feedback path losses.
Figure 1. Basic Circuit and Loop Gain
V
i
V
o
A
B
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