Z86D990 Datasheet, PDF (101/102 Pages) Zilog, Inc. – Low-Voltage Micro controllers with ADC

Z86D990/Z86D991 OTP and Z86L99X ROM
Low-Voltage Microcontrollers with ADC
94
Design Considerations
The Z8 uses a Pierce oscillator with an internal feedback circuit. The advantages
of this circuit are low cost, large output signal, low-power level in the crystal, stabil-
ity with respect to V_CC and temperature, and low impedances (not disturbed by
stray effects.)
One drawback is the requirement for high gain in the amplifier to compensate for
feedback path losses. Traces connecting crystal, capacitors, and the Z8 oscillator
pins must be as short and wide as possible. Short and wide traces reduce para-
sitic inductance and resistance. The components (capacitors, crystal, and resis-
tors) must be placed as close as possible to the oscillator pins of the Z8.
The traces from the oscillator pins of the integrated circuit (IC) and the ground
side of the lead capacitors must be guarded from all other traces (clock, V_CC, and
system ground) to reduce cross-talk and noise injection. Guarding the traces is
usually accomplished 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 capacitors must be
connected to a single trace to the Z8 V_SS (GND) pin. It must not be shared with
any other system ground trace or components except at the Z8 device V_SS pin.
Not sharing the ground side of the oscillator lead capacitors is to prevent differen-
tial system ground noise injection into the oscillator.
PS003807-1002
PRELIMINARY