System Overview
3-21
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
Am186EM/
200 pF
Note 1
X1
b. Crystal Configuration
a. Inverting Amplifier Configuration
C
1
C
2
Crystal
C
1
C
2
Microcontroller
X2
Am188EM
Note 1: Use for Third Overtone Mode
XTAL Frequency
L1 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%
Summary of Contents for AM186EM
Page 1: ...Am186 EM and Am188 EM Microcontrollers User s Manual...
Page 4: ...iv...
Page 12: ...Table of Contents xii...
Page 62: ...Peripheral Control Block 4 10...
Page 76: ...Chip Select Unit 5 14...
Page 122: ...Timer Control Unit 8 8...
Page 136: ...DMA Controller 9 14...
Page 144: ...Asynchronous Serial Port 10 8...
Page 158: ...Programmable I O Pins 12 6...
Page 186: ...Index I 12...