Intel 80C186EA, User Manual

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5-5
CLOCK GENERATION AND POWER MANAGEMENT
5.1.1.2 Selecting Crystals
When specifying crystals, consider these parameters:
•
Resonance and Load Capacitance — Crystals carry a parallel or series resonance specifi-
cation. The two types do not differ in construction, just in test conditions and expected
circuit application. Parallel resonant crystals carry a test load specification, with typical
load capacitance values of 15, 18 or 22 pF. Series resonant crystals do not carry a load
capacitance specification. You may use a series resonant crystal with the microprocessor,
even though the circuit is parallel resonant. However, it will vibrate at a frequency slightly
(on the order of 0.1%) higher than its calibration frequency.
•
Vibration Mode — The vibration mode is either fundamental or third overtone. Crystal
thickness varies inversely with frequency. Vendors furnish third or higher overtone crystals
to avoid manufacturing very thin, fragile quartz crystal elements. At a given frequency, an
overtone crystal is thicker and more rugged than its fundamental mode counterpart. Below
20 MHz, most crystals are fundamental mode. In the 20 to 32 MHz range, you can purchase
both modes. You must know the vibration mode to know whether to add the LC circuit at
OSCOUT.
•
Equivalent Series Resistance (ESR) — ESR is proportional to crystal thickness, inversely
proportional to frequency. A lower value gives a faster startup time, but the specification is
usually not important in microprocessor applications.
•
Shunt Capacitance — A lower value reduces ESR, but typical values such as 7 pF will work
fine.
•
Drive Level — Specifies the maximum power dissipation for which the manufacturer
calibrated the crystal. It is proportional to ESR, frequency, load and V
CC
. Disregard this
specification unless you use a third overtone crystal whose ESR and frequency will be
relatively high. Several crystal manufacturers stock a standard microprocessor crystal line.
Specifying a “microprocessor grade” crystal should ensure that the rated drive level is a
couple of milliwatts with 5-volt operation.
•
Temperature Range — Specifies an operating range over which the frequency will not vary
beyond a stated limit. Specify the temperature range to match the microprocessor
temperature range.
•
Tolerance — The allowable frequency deviation at a particular calibration temperature,
usually 25° C. Quartz crystals are more accurate than microprocessor applications call for;
do not pay for a tighter specification than you need. Vendors quote frequency tolerance in
percentage or parts per million (ppm). Standard microprocessor crystals typically have a
frequency tolerance of 0.01% (100 ppm). If you use these crystals, you can usually
disregard all the other specifications; these crystals are ideal for the 80C186 Modular Core
family.