Crystals And Resonators - ZiLOG Z8 User Manual

ZiLOG

3.4.4 Crystals and Resonators

Crystals and ceramic resonators (Figure 3-7) should have the
following characteristics to ensure proper oscillator operation:
Crystal Cut AT (crystal only)
Mode Parallel, Fundamental Mode
Crystal Capacitance <7pF
Load Capacitance 10pF < CL < 220 pF,
15 typical
Resistance 100 ohms max
Depending on operation frequency, the oscillator may require
the addition of capacitors C1 and C2 (shown in Figures 3-7). The
capacitance values are dependent on the manufacturer's crystal
specifications.
Z8
XTAL1
R
C1
Figure 3-7. Crystal/Ceramic Resonator Oscillator
C1
C2
Figure 3-8. LC Clock
UM001601-0803
V
SS
XTAL2
R
F
D
C2
XTAL1
Z8
L
V
SS
XTAL2
In most cases, the R is 0 Ohms and R
D
mined and specified by the crystal/ceramic resonator manufac-
turer. The R can be increased to decrease the amount of drive
D
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 can be used to improve the start-up of the crys-
F
tal/ceramic resonator. The Z8 oscillator already has an internal
shunt resistor in parallel to the crystal/ceramic resonator.
Figure 3-9. External Clock
It is recommended in Figures 3-7, 3-8, and 3-9 to connect the
load capacitor ground trace directly to the V
®
Z8 . This ensures that no system noise is injected into the Z8
clock. This trace should not be shared with any other compo-
nents except at the V pin of the Z8.
SS
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
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 diode to V
XTAL1. It is recommended that in applications where the Z8 is
exposed to much system noise, a diode from XTAL1 to V
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 series combination
of C and C , including all parasitics (PCB and holder).
1 2
Z8 Microcontrollers
Clock
is infinite. It is deter-
F
XTAL1
Z8
V
SS
XTAL2
(GND) pin of the
SS
on the
CC
is not on
CC
CC
be
3-5