Z85233 Datasheet, PDF (292/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

Application Note
On-Chip Oscillator Design
Load Capacitors
Frequency Tolerance (initial temperature and aging).
In the selection of load caps it is understood that parasitics
are always included.
Initial tolerance is typically ±.01%. Temperature tolerance
is typically ±.005% over the temp range (-30 to +100
degrees C). Aging tolerance is also given, typically
1
Upper Limits. If the load caps are too large, the oscillator ^±.005%.
will not start because the loop gain is too low at the
operating frequency. This is due to the impedance of the
load capacitors. Larger load caps produce a longer
Holder. Typical holder part numbers are HC6, 18,
25, 33, 44.
startup.
Shunt Capacitance. (Cs) typically <7 pf.
Lower Limits. If the load caps are too small, either the
oscillator will not start (due to inadequate phase shift
around the loop), or it will run at a 3rd, 5th, or 7th overtone
frequency (due to inadequate suppression of higher
overtones).
Capacitor Type and Tolerance. Ceramic caps of ±10%
tolerance should be adequate for most applications.
Ceramic vs. Quartz. Manufacturers of ceramic resonators
generally specify larger load cap values than quartz crystals.
Quartz C is typically 15 to 30 pF and ceramic typically 100 pF.
Mode. Typically the mode (fundamental, 3rd or 5th
overtone) is specified as well as the loading configuration
(series vs. parallel).
The ceramic resonator equivalent circuit is the same as
shown in Figure 4. The values differ from those specified
in the theory section. Note that the ratio of L/C is much
lower than with quartz crystals. This gives a lower Q which
allows a faster startup and looser frequency tolerance
(typically ±0.9% over time and temperature) than quartz.
Layout
Summary. For reliable and fast startup, capacitors should
be as small as possible without resulting in overtone
operation. The selection of these capacitors is critical and
all of the factors covered in this note should be considered.
Feedback Element
The following text describes the specific parameters of a
typical crystal:
The following text explains trace layout as it affects the
various stray capacitance parameters (Figure 9).
Traces and Placement. Traces connecting crystal, caps,
and the IC oscillator pins should be as short and wide as
possible (this helps reduce parasitic inductance and
resistance). Therefore, the components (caps and crystal)
should be placed as close to the oscillator pins of the IC as
possible.
Drive Level. There is no problem at frequencies greater
than 1 MHz and V_CC = 5V since high frequency AT cut
crystals are designed for relatively high drive levels (5-10
mw max).
A typical calculation for the approximate power dissipated
in a crystal is:
Grounding/Guarding. The traces from the oscillator pins
of the IC should be guarded from all other traces (clock,
V_CC, address/data lines) to reduce crosstalk. This is
usually accomplished by keeping other traces away from
the oscillator circuit and by placing a ground ring around
the traces/components (Figure 9).
P = 2R (π x f x C x V_CC)₂
Where. R = crystal resistance of 40 ohms, C = C1 + Co =
20 pF. The calculation gives a power dissipation of 2 mW
at 16 MHz.
Series Resistance. Lower series resistance gives better
performance but costs more. Higher R results in more
power dissipation and longer startup, but can be
compensated by reduced C1 and C2. This value ranges
from 200 ohms at 1 MHz down to 15 ohms at 20 MHz.
Frequency. The frequency of oscillation in parallel
resonant circuits is mostly determined by the crystal
(99.5%). The external components have a negligible effect
(0.5%) on frequency. The external components (C1,C2)
and layout are chosen primarily for good startup and
reliability reasons.
Measurement and Observation
Connection of a scope to either of the circuit nodes is likely
to affect operation because the scope adds 3-30 pF of
capacitance and 1M-10M ohms of resistance to the circuit.
Indications of an Unreliable Design
There are two major indicators which are used in working
designs to determine their reliability over full lot and
temperature variations. They are:
Start Up Time. If start up time is excessive, or varies
widely from unit to unit, there is probably a gain problem.
C1/C2 needs to be reduced; the amplifier gain is not
adequate at frequency, or crystal Rs is too large.
UM010901-0601
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