Z8
®
CPU
User Manual
UM001604-0108 Clock
31
In Figure 18 through Figure 20, Zilog
®
recommends that you connect the load capacitor
ground trace directly to the V
SS
(GND) pin of the Z8
®
CPU to ensure that no system noise
is injected into the Z8 clock. This trace should not be shared with any other components
except at the V
SS
pin of the Z8 CPU.
In some cases, the Z8 CPU’s XTAL1 pin also functions as one of the EPROM high-volt-
age mode programming pins or as a special factory test pin. In this case, applying 2 V
above V
CC
on the XTAL1 pin causes the device to enter one of these modes. Because this
pin accepts high voltages to enter these respective modes, the standard input protection
diode to V
CC
is not on XTAL1. Zilog recommends that in applications where the Z8 CPU
is exposed to much system noise, a diode from XTAL1 to V
CC
be used to prevent acciden-
tal enabling of these modes. This diode does not affect the crystal/ceramic resonator oper-
ation.
A parallel resonant crystal or resonator data sheet specifies a load capacitor value that is
the series combination of C
1
and C
2
, including all parasitics (PCB and holder).
LC Oscillator
The Z8 CPU oscillator can use a LC network to generate a XTAL clock (see Figure 19 on
page 30).
The frequency stays stable over V
CC
and temperature. The oscillation frequency is deter-
mined by following equation.
where L is the total inductance including parasitics and C
T
is the total series capacitance
including the parasitics.
Figure 20. External Clock
XTAL2
Z8
®
CPU
V
SS
XTAL1
Note:
Frequency
1
2π LC
T
()
12⁄
--------------------------------------=
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