80C186 and 80C188 Integrated
This document amends the 80C186 and 80C188 Integrated 16-Bit Microprocessors Data Book, order #16514D, and
replaces the discontinued 80C186/80C188 CMOS High-Integration 16-Bit Microprocessors Amendment (specifica-
tions for the 20-MHz industrial operating range). This amendment consists of two parts:
n Clock generation information changes for the 80C186 and 80C188 microcontrollers. If the guidelines in this bulletin
are not followed, you may experience problems with clock start-up.
n Industrial operating information at 20 MHz. This is the same information that was published in the discontinued
80C186/80C188 CMOS High-Integration 16-Bit Microprocessors Amendment.
CLOCKING INFORMATION CHANGES
Crystal-Driven Clock Source
The internal oscillator circuit of the microcontroller is
designed to function with a parallel resonant
fundamental or third-overtone crystal. The 80C186 and
80C188 microprocessors use a crystal frequency that
is twice the processor frequency. AMD does not
recommend that you replace a crystal with an LC or RC
equivalent for any member of the Am186™ family.
The X1 and X2 signals are connected to an internal
inverting amplifier (oscillator) that provides, along with
the external feedback loading, the necessary phase
shift (Figure 1 on page 2). In such a positive feedback
circuit, the inverting amplifier has an output signal (X2)
180 degrees out of phase of the input signal (X1). The
external feedback network provides an additional 180
degree phase shift. In an ideal system, the input to X1
has 360 or zero degrees of phase shift.
The external feedback network is designed to be as
close as possible to ideal. If the feedback network is not
providing necessary phase shift, negative feedback
dampens the output of the amplifier and negatively
affects the operation of the clock generator. Values for
the loading on X1 and X2 must be chosen to provide
the necessary phase shift and crystal operation.
Selecting a Crystal
When selecting a crystal, you should always specify the
load capacitance (CL). This value can cause variance in
the oscillation frequency from the desired specified value
(resonance). The load capacitance and the loading of the
feedback network have the following relationship:
CL = (( C1 • C2)/( C1+ C2)) + CS
where CS is the stray capacitance of the circuit. Placing
the crystal and CL in series across the inverting
amplifier and tuning these values (C1, C2) allows the
crystal to oscillate at resonance. This relationship is
true for both fundamental and third-overtone operation.
Finally, there is a relationship between C1and C2. To
enhance the oscillation of the inverting amplifier, these
values must be offset with the larger load on the output
(X2). Equal values of these loads tend to balance the
poles of the inverting amplifier.
The characteristics of the inverting amplifier set limits
on the following parameters for crystals:
ESR (Equivalent Series Resistance) ........... 40 Ω Max
Drive Level .................................................. 1 mW Max
The recommended range of values for C1and C2 are
C1.............................................................15 pF ± 20%
C2.............................................................22 pF ± 20%
You must determine the specific values for C1 and C2.
The values are dependent on the characteristics of the
chosen crystal and board design. The C1 and C2
values include the stray capacitances of the design.
Figure 1 on page 2 shows the correct connection of the
oscillator configurations. Figure 1a shows the inverting
amplifier configuration. This is the equivalent circuitry
with the inverter integrated into the microcontroller.
Figure 1b shows the crystal configuration. The diagram
shows the correct connection for third-overtone
crystals. The fundamental mode crystals do not require
the L1 or the 200-pF capacitor. Figure 1c shows the
recommended crystal mode based on the crystal
frequency. The 80C186 and 80C188 microprocessors
use a crystal twice the CPU frequency and can use
either fundamental or third-overtone mode crystals,
depending on the CPU frequency.
This document contains information on a product under development at Advanced Micro Devices. The information
is intended to help you evaluate this product. AMD reserves the right to change or discontinue work on this product
Publication# 16514 Rev: D Amendment/1
Issue Date: October 1998