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參數(shù)資料
| 型號： | Z80B-CTC |
| 英文描述： | Z8 Microcontrollers |
| 中文描述： | Z8微控制器 |
| 文件頁數(shù)： | 35/222頁 |
| 文件大?。?/td> | 1595K |
| 代理商： | Z80B-CTC |
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Z8 Microcontrollers
ZiLOG
Clock
UM001600-Z8X0599
3-3
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 respect to V
CC
and temperature, and low im-
pedances (not disturbed by stray effects).
One draw back is the need for high gain in the amplifier to
compensate for feedback path losses. The oscillator am-
plifies its own noise at start-up until it settles at the frequen-
cy that satisfies the gain/phase requirements A x B = 1,
where A = V
0
/V
is the gain of the amplifier and B = V
/V
0
is
the gain of the feedback element. The total phase shift
around the loop is forced to zero (360 degrees). Since 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 tran-
sition.
Capacitor C
2
combined with the amplifier output resistance
provides a small phase shift. It will also provide some at-
tenuation of overtones.
Capacitor C
1
combined with the crystal resistance pro-
vides additional phase shift.
C
1
and C
2
can affect the start-up time if they increase dra-
matically 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.
It is recommended for fast and reliable oscillator start-up
(over the manufacturing process range) that the load ca-
pacitors be sized as low as possible without resulting in
overtone operation.
3.4.1 Layout
Traces connecting crystal, caps, and the Z8 oscillator pins
should be as short and wide as possible. This reduces par-
asitic inductance and resistance. The components (caps,
crystal, resistors) 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 trac-
es (clock, V
CC
, address/data lines, system ground) to re-
duce cross talk and noise injection. This is usually accom-
plished by keeping other traces and system ground trace
planes away from the oscillator circuit and by placing a Z8
device V
SS
ground ring around the traces/components.
The ground side of the oscillator lead caps should be con-
nected to a single trace to the Z8 V
SS
(GND) pin. It should
not be shared with any other system ground trace or com-
ponents except at the Z8 device V
SS
pin. This is to prevent
differential system ground noise injection into the oscillator
3.4.2 Indications of an Unreliable Design
There are two major indicators that are used in working de-
signs to determine their reliability over full lot and temper-
ature variations. They are:
Start-up Time. If start -up time is excessive, or varies wide-
ly 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
CC
. This indicates there is adequate
gain in the 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 oscil-
lation is achieved. A signal of less than 2.5 volts peak-to-
peak is an indication that low gain may be a problem. Ei-
ther C
1
or C
2
should be made smaller or a low-resistance
crystal should be used.
Figure 3-5. Pierce Oscillator with Internal
Feedback Circuit
XTAL2
Z8
V
SS
XTAL1
C1
C2
R
I
V
1
A
V
0
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