SC2200 datasheet(94/433 Pages) NSC

94 / 433 page

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Revision 3.0

General Configuration Block (Continued)

3.5.1

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 3-3 for the recommended external circuit and Table

3-5 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 =10pF, CPARASITIC =8.2 pF

C1 =3.6 pF, C2 =3.6 pF

Example 2:

Crystal CL =20pF, CPARASITIC =8 pF

C1 =24pF, C2 =24pF

Figure 3-3. Recommended Oscillator External

Circuitry

Internal

External

X27I

X27O

To other

modules

Table 3-5. 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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