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32579B
4.5.1
27 MHz Crystal Oscillator
The internal oscillator employs an external crystal con-
nected to the on-chip amplifier. The on-chip amplifier is
accessible on the X27I input and X27O output signals. See
Figure 4-3 for the recommended external circuit and Table
4-5 for a list of the circuit components.
Choose C
and C
capacitors to match the crystal's load
1
2
capacitance. The load capacitance C
is comprised of C
in series with C
1
parasitic capacitance of the circuit. The parasitic capaci-
tance is caused by the chip package, board layout and
socket (if any), and can vary from 0 to 10 pF. The rule of
thumb in choosing these capacitors is:
C
= (C
* C
) / (C
+ C
L
1
2
1
Example 1:
Crystal C
= 10 pF, C
L
PARASITIC
C
= 3.6 pF, C
= 3.6 pF
1
2
Example 2:
Crystal C
= 20 pF, C
L
PARASITIC
C
= 24 pF, C
= 24 pF
1
2
Component
Parameters
Crystal
Resonance Frequency
Type
Serial Resistance
Shunt Capacitance
Load Capacitance, C
Temperature Coefficient
Resistor R
Resistance
1
1
Resistance
Resistor R
2
1
Capacitance
Capacitor C
1
1
Capacitance
Capacitor C
2
1.
The value of these components is recommended. It should be tuned according to crystal and board parameters.
84
"seen" by crystal Y
L
and in parallel with the
2
) + C
2
PARASITIC
= 8.2 pF
= 8 pF
Table 4-5. Crystal Oscillator Circuit Components
L
X27I
C
Figure 4-3. Recommended Oscillator External
Values
27.00 MHz Parallel mode
AT-cut or BT-cut
40 Ω
7 pF
10-20 pF
User-defined
20 MΩ
100 Ω
3-24 pF
3-24 pF
AMD Geode™ SC1200/SC1201 Processor Data Book
General Configuration Block
To other
modules
Internal
External
X27O
R
1
R
2
Y
C
1
2
Circuitry
Tolerance
50 PPM or better
Max
Max
5%
5%
5%
5%
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