AM186EM-33VCSLASHW datasheet(41/98 Pages) AMD

41 / 98 page

Am186/188EM and Am186/188EMLV Microcontrollers

P R E L IMINAR Y

CLOCK AND POWER MANAGEMENT

The clock an d po wer m an age me nt u nit of the

Am186EM and Am188EM microcontrollers includes a

phase-locked loop (PLL) and a second programmable

system clock output (CLKOUTB).

Phase-Locked Loop (PLL)

In a traditional 80C186/188 design, the crystal frequency is

twice that of the desired internal clock. Because of the

internal PLL on the Am186EM and Am188EM

microcontrollers, the internal clock generated by the

Am186EM and Am188EM microcontrollers (CLKOUTA) is

the same frequency as the crystal. The PLL takes the

crystal inputs (X1 and X2) and generates 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 by an

on-chip power-on reset (POR) circuit.

Crystal-Driven Clock Source

The internal oscillator circuit of the Am186EM and

Am188EM 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 signals X1 and X2 are connected to an internal in-

verting amplifier (oscillator) which 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).

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 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:

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 os-

cillate at resonance. This relationship is true for both fun-

damental and third-overtone operation. Finally, there is a

relationship between C1 and C2. To enhance the oscilla-

tion of the inverting amplifier, these values need to be off-

set with the larger load on the output (X2). Equal values of

these loads will 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)........... 80 ohm max

Drive Level.....................................................................................1 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 characteristics of

the chosen crystal and board design.

Figure 8.

Am186EM and Am188EM Microcontrollers Oscillator Configurations

(C1 ⋅ C2)

CL =

(C1 + C2)

+ CS

Crystal

Am186EM

200 pF

Note 1

Note 1: Use for Third Overtone Mode

XTAL Frequency L1 Value (Max)

20 MHz

µH ±20%

25 MHz

8.2

µH ±20%

33 MHz

4.7

µH ±20%

40 MHz

3.0

µH ±20%

b. Crystal Configuration

a. Inverting Amplifier Configuration

Crystal

Microcontroller

AM186EM-33VC/W Datasheet(PDF) 41 Page - Advanced Micro Devices

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