32581C

General Configuration Block

4.5.127 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 4-3for the recommended external circuit and Table 4-5for 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:

 

 

To other

 

 

modules

 

 

Internal

X27I

 

External

R1

X27O

 

 

R2

 

 

C1

Y

C2

Figure 4-3. Recommended Oscillator External

Circuitry

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

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

40 Ω

Max

 

 

 

 

 

Shunt Capacitance

7 pF

Max

 

 

 

 

 

Load Capacitance, CL

10-20 pF

 

 

 

 

 

 

Temperature Coefficient

User-defined

 

 

 

 

 

Resistor R1

Resistance

20 MΩ

5%

Resistor R21

Resistance

100 Ω

5%

Capacitor C11

Capacitance

3-24 pF

5%

Capacitor C21

Capacitance

3-24 pF

5%

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

82

AMD Geode™ SC3200 Processor Data Book

Page 82
Image 82
AMD SC3200 manual 1 27 MHz Crystal Oscillator, Crystal Oscillator Circuit Components, Component Parameters Values Tolerance