AM186 Datasheet, PDF (45/112 Pages) Advanced Micro Devices – High-Performance, 80C186- and 80C188-Compatible, 16-Bit Embedded Microcontrollers with RAM

Crystal-Driven Clock Source
The internal oscillator circuit of the Am186ER and
Am188ER microcontrollers is designed to function with
a parallel-resonant fundamental mode crystal. Be-
cause of the PLL, the crystal frequency can be twice,
equal to, or one quarter of the processor frequency. Do
not replace a crystal with an LC or RC equivalent. See
Figure 8 for a diagram of oscillator configurations.
where C_S is the stray capacitance of the circuit. Placing
the crystal and C_L in series across the inverting ampli-
fier and tuning these values (C₁, C₂) allows the crystal
to oscillate at resonance. 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 will tend to balance the poles of the invert-
ing amplifier.
The X1 and X2 signals are connected to an internal in-
verting amplifier (oscillator) that provides, along with
the external feedback loading, the necessary phase
The characteristics of the inverting amplifier set limits
on the following parameters for crystals:
shift. In such a positive feedback circuit, the inverting ESR (Equivalent Series Resistance)60-ohm max
amplifier has an output signal (X2) 180 degrees out of
phase of the input signal (X1).
Drive Level .......................... 500-mW max
The external feedback network provides an additional
180-degree phase shift. In an ideal system, the input to
The recommended range of values for C₁ and C₂ are
as follows:
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 will
dampen the output of the amplifier and negatively af-
fect the operation of the clock generator. Values for the
loading on X1 and X2 must be chosen to provide the
T _{necessary phase shift and crystal operation.}
Selecting a Crystal
When selecting a crystal, the load capacitance should
F _{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:
A _CL
=
(C₁
(C₁
⋅ C₂)
+ C₂)
+ C_S
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.
External Source Clock
Alternately, the internal oscillator can be driven by an
external clock source. The external clock source
should be connected to the input of the inverting ampli-
fier (X1) with the output (X2) left unconnected. X1 and
X2 are not 5-V tolerant and X1 has a maximum input
equal to V_CC.
R_X1
D ^Oscillator
C₁
X1
Crystal
Oscillator
X2
To PLL
C₂
X2
To PLL
Am188ER/
Am188ER/
Am186ER
Am186ER
Microcontroller
Microcontroller
a. External Clock Configuration
b. Crystal Configuration
Notes:
X1 and X2 are not 5-V tolerant. The X1 maximum input is V_CC.
Figure 8. Am186™ER and Am188™ER Microcontrollers Oscillator Configurations
Am186^TMER and Am188^TMER Microcontrollers Data Sheet
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