AM186 datasheet(45/112 Pages) AMD

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Am186TMER and Am188TMER Microcontrollers Data Sheet

DRAF

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 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.

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:

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.

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:

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 VCC.

Figure 8. Am186™ER and Am188™ER Microcontrollers Oscillator Configurations

(C1 ⋅ C2)

CL =

(C1 + C2)

+ CS

Crystal

b. Crystal Configuration

To PLL

Oscillator

a. External Clock Configuration

Microcontroller

Am188ER/

To PLL

Oscillator

Am186ER

Microcontroller

Am188ER/

Am186ER

Notes:

X1 and X2 are not 5-V tolerant. The X1 maximum input is VCC.

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