AMD Am186 ES, Руководство пользователя, Страница: 58 / 192

System Overview 3-26
3.4.2.1 Selecting a Crystal
When selecting a crystal, the load capacitance should always be specified (C
L
). 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 C
S
is the stray capacitance of the circuit. Placing the crystal and C
L
in series across
the inverting amplifier and tuning these values (C
1
, C
2
) allows the crystal to oscillate at
resonance. This relationship is true for both fundamental and third-overtone operation.
Finally, there is a relationship between C
1
and C
2
. 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.
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 C
1
and C
2
are as follows:
C
1
....................................................................... 15 pF
±
20%
C
2
....................................................................... 22 pF
±
20%
The specific values for C
1
and C
2
must be determined by the designer and are dependent
on the characteristics of the chosen crystal and board design.
Figure 3-5 Oscillator Configurations
(C
1
⋅
C
2
)
C
L
=
(C
1
+ C
2
)
+ C
S
Crystal
Am186ES
200 pF
Note 1
X1
b. Crystal Configuration
a. Inverting Amplifier Configuration
C
1
C
2
Crystal
C
1
C
2
X2
Microcontroller
Note 1: Use for Third Overtone Mode
XTAL FrequencyL1 Value (Max)
20 MHz
12
µ
H
±
20%
25 MHz 8.2
µ
H
±
20%
33 MHz 4.7
µ
H
±
20%
40 MHz 3.0
µ
H
±
20%