88 AMD Geode™ SC2200 Processor Data Book
General Configuration Block
32580B
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 C1 and C2 capacitors to match the crystal’s load
capacitance. The load capacitance CL “seen” by crystal Y
is comprised of C1 in series with C2 and in parallel with the
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:
CL = (C1 * C2) / (C1 + C2) + CPARASITIC
Example 1:
Crystal CL = 10 pF, CPARASITIC = 8.2 pF
C1 = 3.6 pF, C2 = 3.6 pF
Example 2:
Crystal CL = 20 pF, CPARASITIC = 8 pF
C1 = 24 pF, C2 = 24 pF
Figure 4-3. Recommended Oscillator External Circuitry
Internal
External
R2
X27I X27O
C1C2
R1
Y
To other
modules
Table 4-5. Crystal Oscillator Circuit Components
Component Parameters Values Tolerance
Crystal Resonance Frequency 27.00 MHz Parallel mode 50 PPM or better
Type AT-cut or BT-cut
Serial Resistance 40ΩMax
Shunt Capacitance 7 pF Max
Load Capacitance, CL10-20 pF
Temperature Coefficient User-defined
Resistor R1Resistance 20MΩ5%
Resistor R21Resistance 100Ω5%
Capacitor C11Capacitance 3-24pF 5%
Capacitor C21Capacitance 3-24pF 5%
1. The value of these components is recommended. It should be tuned according to crystal and board parameters.