Z8E001 Datasheet, PDF (23/49 Pages) Zilog, Inc. – CMOS OTP Microcontroller

Zilog
OSCILLATOR OPERATION
The Z8E001 MCU uses a Pierce oscillator with an internal
feedback (Figure 13). 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 = V₀/V_i is the gain of the amplifier and B = V_i/V₀ 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 C₂ combined with the amplifier output resistance
provides a small phase shift. It will also provide some at-
tenuation of overtones.
Capacitor C₁ combined with the crystal resistance pro-
vides additional phase shift.
C₁ and C₂ can affect the start-up time if they increase dra-
matically in size. As C₁ and C₂ increase, the start-up time
increases until the oscillator reaches a point where it does
not start up any more.
Z8E001
CMOS OTP Microcontroller
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.
Z8E001
V_SS
A
V₁
R_I
V₀
XTAL1
C1
XTAL2
C2
Figure 15. Pierce Oscillator with Internal Feedback
Circuit
Layout
Traces connecting crystal, caps, and the Z8E001 oscillator
pins should be as short and wide as possible. This reduces
parasitic inductance and resistance. The components
(caps, crystal, resistors) should be placed as close as pos-
sible to the oscillator pins of the Z8E001.
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
Z8E001 device V_SS ground ring around the traces/compo-
nents. The ground side of the oscillator lead caps should
be connected to a single trace to the Z8E001 V_SS (GND)
pin. It should not be shared with any other system ground
trace or components except at the Z8E001 device V_SS pin.
This is to prevent differential system ground noise injection
into the oscillator (Figure 14).
DS97Z8X1300
PRELIMINARY
23