Oscillator Operation; Layout; Indications Of An Unreliable Design - ZiLOG Z8 User Manual

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3.4 OSCILLATOR OPERATION

®
The Z8 MCU uses a Pierce oscillator with an internal feedback
(Figure 3-5). The advantages of this circuit are low cost, large
output signal, low-power level in the crystal, stability with re-
spect to V and temperature, and low impedances (not dis-
CC
turbed by stray effects).
One draw back is the need for high gain in the amplifier to com-
pensate for feedback path losses. The oscillator amplifies its own
noise at start-up until it settles at the frequency that satisfies the
gain/phase requirements A x B = 1, where A = V
of the amplifier and B = V
I
ment. The total phase shift around the loop is forced to zero (360
degrees). Since VIN must be in phase with itself, the amplifi-
er/inverter provides 180 degree phase shift and the feedback el-
ement 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 transition.
Capacitor C combined with the amplifier output resistance pro-
2
vides a small phase shift. It will also provide some attenuation of
overtones.
Capacitor C combined with the crystal resistance provides ad-
1
ditional phase shift.

3.4.1 Layout

Traces connecting crystal, caps, and the Z8 oscillator pins
should be as short and wide as possible. This reduces parasitic
inductance and resistance. The components (caps, crystal, resis-
tors) should be placed as close as possible to the oscillator pins
of the Z8.
The traces from the oscillator pins of the IC and the ground side
of the lead caps should be guarded from all other traces (clock,
V , address/data lines, system ground) to reduce cross talk and
CC
noise injection. This is usually accomplished by keeping other
traces and system ground trace planes away from the oscillator
circuit and by placing a Z8 device V
traces/components. The ground side of the oscillator lead caps
should be connected to a single trace to the Z8 V
It should not be shared with any other system ground trace or
components except at the Z8 device V
differential system ground noise injection into the oscillator
(Figure 3-6).
UM001601-0803
/V is the gain
0
I
/V is the gain of the feedback ele-
0
ground ring around the
SS
(GND) pin.
SS
pin. This is to prevent
SS
C and C can affect the start-up time if they increase dramati-
1 2
cally in size. As C and C
1 2
until the oscillator reaches a point where it does not start up any
more.
It is recommended for fast and reliable oscillator start-up (over
the manufacturing process range) that the load capacitors be
sized as low as possible without resulting in overtone operation.
Z8
A
R
V
I
1
XTAL1
C1
Figure 3-5. Pierce Oscillator with Internal Feedback

3.4.2 Indications of an Unreliable Design

There are two major indicators that are used in working designs
to determine their reliability over full lot and temperature varia-
tions. They are:
Start-up Time. If start -up time is excessive, or varies widely
from unit to unit, there is probably a gain problem. C1/C2 needs
to be reduced; the amplifier gain is not adequate at frequency, or
crystal Rs is too large.
Output Level. The signal at the amplifier output should swing
from ground to V . This indicates there is adequate gain in the
CC
amplifier. As the oscillator starts up, the signal amplitude grows
until clipping occurs, at which point the loop gain is effectively
reduced to unity and constant oscillation is achieved. A signal of
less than 2.5 volts peak-to-peak is an indication that low gain
may be a problem. Either C
low-resistance crystal should be used.
Z8 Microcontrollers
increase, the start-up time increases
V
SS
V
0
XTAL2
C2
Circuit
or C should be made smaller or a
1 2
Clock
3-3