P R E L I M I N A R Y
CLOCK AND POWER MANAGEMENT
The clock and power management unit of the
Am186ES and Am188ES microcontrollers includes a
phase-locked loop (PLL) and a second programmable
system clock output (CLKOUTB).
the output of the amplifier and negatively affects the op-
eration 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
Phase-Locked Loop (PLL)
When selecting a crystal, the load capacitance should
always be specified (CL). This value can cause vari-
ance in the oscillation frequency from the desired spec-
ified value (resonance). The load capacitance and the
loading of the feedback network have the following re-
lationship:
In a traditional 80C186/188 microcontroller design, the
crystal frequency is twice that of the desired internal
clock. Because of the internal PLL on the Am186ES
and Am188ES microcontrollers, the internal clock gen-
erated by the Am186ES and Am188ES microcontrol-
lers (CLKOUTA) is the same frequency as the crystal.
The PLL takes the crystal inputs (X1 and X2) and gen-
erates a 45–55% (worst case) duty cycle intermediate
system clock of the same frequency. This removes the
need for an external 2x oscillator, reducing system
cost. The PLL is reset during power-on reset by an on-
chip power-on reset (POR) circuit.
(C1 C2)
CL =
+ CS
(C1 + C2)
where CS is the stray capacitance of the circuit. Placing
the crystal and CL in series across the inverting ampli-
fier 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 C1 and C2. To enhance
the oscillation of the inverting amplifier, these values
need to be offset with the larger load on the output (X2).
Equal values of these loads tend to balance the poles
of the inverting amplifier.
Crystal-Driven Clock Source
The internal oscillator circuit of the Am186ES and
Am188ES microcontrollers is designed to function with
a parallel resonant fundamental or third overtone crys-
tal. Because of the PLL, the crystal frequency should
be equal to the processor frequency. Do not replace a
crystal with an LC or RC equivalent.
The characteristics of the inverting amplifier set limits
on the following parameters for crystals:
The signals X1 and X2 are connected to an internal in-
verting amplifier (oscillator) that provides, along with
the external feedback loading, the necessary phase
shift (Figure 8). In such a positive feedback circuit, the
inverting amplifier has an output signal (X2) 180 de-
grees out of phase of the input signal (X1).
ESR (Equivalent Series Resistance) ......40 Ω max
Drive Level..............................................1 mW max
The recommended range of values for C1 and C2 are
as follows:
The external feedback network provides an additional
180-degree phase shift. In an ideal system, the input to
X1 will have 360 or zero degrees of phase shift. The ex-
ternal feedback network is designed to be as close to
ideal as possible. If the feedback network is not provid-
ing necessary phase shift, negative feedback dampens
C1 ..................................................................15 pF ± 20%
C2 ..................................................................22 pF ± 20%
The specific values for C1 and C2 must be determined
by the designer and are dependent on the characteris-
tics of the chosen crystal and board design.
C1
X1
Crystal
X2
Crystal
C2
Am186ES
C1
C2
Microcontroller
Note 1
Note 1: Use for Third Overtone Mode
XTAL Frequency L1 Value (Max)
a. Inverting Amplifier Configuration
200 pF
b. Crystal Configuration
20 MHz
25 MHz
33 MHz
40 MHz
12 µH ±20%
8.2 µH ±20%
4.7 µH ±20%
3.0 µH ±20%
Figure 8. Am186ES and Am188ES Microcontrollers Oscillator Configurations
Am186/188ES and Am186/188ESLV Microcontrollers
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