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. 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.
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.
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
shift. 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 characteristics of the inverting amplifier set limits
on the following parameters for crystals:
ESR (Equivalent Series Resistance)60-ohm max
Drive Level .......................... 500-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 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
necessary phase shift and crystal operation.
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
Selecting a Crystal
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:
equal to VCC
.
(C1 C2)
(C1 + C2)
CL =
+ CS
C1
C2
X1
Crystal
X2
X1
X2
Oscillator
To PLL
Oscillator
To PLL
Am188ER/
Am186ER
Am188ER/
Am186ER
Microcontroller
Microcontroller
a. External Clock Configuration
b. Crystal Configuration
Notes:
X1 and X2 are not 5-V tolerant. The X1 maximum input is VCC
.
Figure 8. Am186™ER and Am188™ER Microcontrollers Oscillator Configurations
Am186TMER and Am188TMER Microcontrollers Data Sheet
45
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