APPLICATION
BRIEF
AB-37
February 1996
80186/80188 EFI Drive and
Oscillator Operation
STEVE FARRER
APPLICATIONS ENGINEER
Order Number: 270526-001
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COPYRIGHT ©INTEL CORPORATION, 1996
80186/80188 EFI DRIVE
AND OSCILLATOR
OPERATION
CONTENTS PAGE
EFI Operation ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 1
Crystal Operation ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ 1
AB-37
There has been some confusion in the past regarding
the correct input for EFI (External Frequency Input)
use and what parameters should be used for crystal
selection. This Application Brief discusses the trade-
offs with each input so that one can decide which input
suits his design and also lists the parameters for crystal
selection.
EFI Operation
The oscillator circuit on the 186/188 is as shown in
Figure 1 (simplified). Either input may be used for an
EFI signal. Using X1 requires very little drive from an
external oscillator since it is essentially the gate of an
NMOS transistor. Clock operation works fine using
this input, but at higher frequencies the stray capaci-
tance on X2 begins to change the duty cycle of the
clock. This will eventually cause the part to fail.
270526–1
NOTE:
Driving X2does not allow compatibility with future
CMOS designs.
Figure 1. Oscillator Circuit on the 186/188
Using X2 as an EFI gives a broader frequency range
but places a more stringent requirement on the drive
capability of the external oscillator. Since X1 is an in-
put, it may be grounded to minimize the capacitance.
This in turn allows for a higher frequency range since
the duty cycle remains closer to 50%. But with X1
grounded, the output of the inverter (which is directly
connected to X2) is always trying to output a high. This
means the oscillator driving X2 must be capable of
sinking up to 15 mA at cold temperatures when trying
to drive it low. If the external oscillator is capable of
supplying 15 mA, then this method is preferred. Other-
wise, X1 should be used as an EFI.
Caution: using X2 for EFI does not allow for CMOS
compatibility at a future date.
Crystal Operation
The oscillator circuit is a single stage amplifier connect-
ed as a Pierce oscillator. There are no passive compo-
nents in the oscillator circuit, only a unique combina-
tion of depletion and enhancement mode FET’s. Char-
acterization of the oscillator circuit showed that opera-
tion was optimum with crystal parameters as follows:
ESR 30 ohms maximum
(Equivalent Series Resistance)
Co (Shunt Capacitance) 7.0 pf max.
C1 (Load Capacity) 20 pf g2pf
Drive Level 1 mW max.
This characterization data was supplied by:
Standard Crystal Corporation
9940 East Baldwin Place
El Monte, CA 91731
(213) 443-2121
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