Z8 Microcontrollers
ZiLOG
Clock
UM001600-Z8X0599
3-5
3.4.4 Crystals and Resonators
Crystals and ceramic resonators (Figure 3-7) should have
the following characteristics to ensure proper oscillator op-
eration:
Depending on operation frequency, the oscillator may re-
quire the addition of capacitors C1 and C2 (shown in Fig-
ures 3-7). The capacitance values are dependent on the
manufacturer’s crystal specifications.
In most cases, the R
D
is 0 Ohms and R
F
is infinite. It is de-
termined and specified by the crystal/ceramic resonator
manufacturer. The R
D
can be increased to decrease the
amount of drive from the oscillator output to the crystal. It
can also be used as an adjustment to avoid clipping of the
oscillator signal to reduce noise. The R
F
can be used to im-
prove the start-up of the crystal/ceramic resonator. The Z8
oscillator already has an internal shunt resistor in parallel
to the crystal/ceramic resonator.
It is recommended in Figures 3-7, 3-8, and 3-9 to connect
the load capacitor ground trace directly to the V
SS
(GND)
pin of the Z8
. This ensures that no system noise is inject-
ed into the Z8 clock. This trace should not be shared with
any other components except at the V
SS
pin of the Z8.
In some cases, the Z8 XTAL1 pin also functions as one of
the EPROM high-voltage mode programming pins or as a
special factory test pin. In this case, applying 2 V above
V
CC
on the XTAL1 pin will cause the device to enter one of
these modes. Since this pin accepts high voltages to enter
these respective modes, the standard input protection di-
ode to V
CC
is not on XTAL1. It is recommended that in ap-
plications where the Z8 is exposed to much system noise,
a diode from XTAL1 to V
CC
be used to prevent accidental
enabling of these modes. This diode will not affect the
crystal/ceramic resonator operation.
Please note that a parallel resonant crystal or resonator
data sheet will specify a load capacitor value that is the se-
ries combination of C
1
and C
2
, including all parasitics (PCB
and holder).
Crystal Cut
Mode
Crystal Capacitance
Load Capacitance
AT (crystal only)
Parallel, Fundamental Mode
<7pF
10pF < CL < 220 pF,
15 typical
100 ohms max
Resistance
Figure 3-7. Crystal/Ceramic Resonator
Oscillator
Figure 3-8. LC Clock
XTAL2
Z8
V
SS
XTAL1
C1
C2
R
F
R
D
XTAL2
Z8
V
SS
XTAL1
C1
C2
L
Figure 3-9. External Clock
XTAL2
Z8
V
SS
XTAL1