SC3200 datasheet(100/447 Pages) AMD

100 / 447 page

100

AMD Geode™ SC3200 Processor Data Book

General Configuration Block

Revision 5.1

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

X27I

X27O

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

Ω

Max

Shunt Capacitance

7 pF

Max

Load Capacitance, CL

10-20 pF

Temperature Coefficient

User-defined

Resistor R1

Resistance

20 M

Ω

Resistor R2

Resistance

100

Ω

Capacitor C1

Capacitance

3-24 pF

Capacitor C2

Capacitance

3-24 pF

The value of these components is recommended. It should be tuned according to crystal and board parameters.

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