Z8
®
CPU
User Manual
UM001604-0108 Clock
27
Oscillator Operation
The Z8
®
CPU uses a Pierce oscillator with an internal feedback (see Figure 16). The
advantages of this circuit are 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
affects).
One drawback is the requirement for high gain in the amplifier to compensate for feedback
path losses. The oscillator amplifies its own noise at start-up until it settles at the fre-
quency that satisfies the gain/phase requirements A x B = 1, where A = V
0
/V
I
is the gain of
the amplifier and B = V
I
/V
0
is the gain of the feedback element. The total phase shift
around the loop is forced to zero (360 degrees). Because VIN must be in phase with itself,
the amplifier/inverter provides 180 degree phase shift and the feedback element is forced
to provide the other 180 degrees of phase shift.
R1 is a resistive component placed from output to input of the amplifier. The purpose of
this feedback is to bias the amplifier in its linear region and to provide the start-up transi-
tion.
Capacitor C
2
combined with the amplifier output resistance provides a small phase shift. It
also provides some attenuation of overtones.
Capacitor C
1
combined with the crystal resistance provides additional phase shift.
C
1
and C
2
can affect the start-up time if they increase dramatically in size. As C
1
and C
2
increase, the start-up time increases until the oscillator reaches a point where it does not
start up any more.
For fast and reliable oscillator start-up over the manufacturing process range, Zilog
®
rec-
ommends that the load capacitors be sized as low as possible without resulting in overtone
operation.
Figure 16. Pierce Oscillator with Internal Feedback Circuit
XTAL2
Z8 CPU
V
SS
XTAL1
C1
C2
R
I
V
1
A
V
0
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