CS5535 datasheet(145/555 Pages) NSC

145 / 555 page

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RTC Functional Description (Continued)

4.14.1 External Use Recommendations

It is recommended that the external components for the

oscillator be connected as illustrated in Figure 4-48. The

recommended specifications for those external compo-

nents are listed in Table 4-29.

Capacitors C1 and C2 should be chosen to match the crys-

tal’s load capacitance. The load capacitance CL “seen” by

the crystal Y is comprised of C1 in series with C2 in parallel

with the parasitic capacitance of the circuit. The parasitic

capacitance is caused by the chip package, board layout,

and socket (if any). The rule of thumb in choosing these

capacitors is:

CL = (C1 * C2)/(C1 + C2) + CPARASITIC

C1 can be trimmed to achieve precisely 32.768 kHz. To

achieve high time accuracy, use crystal and capacitors with

low tolerance and temperature coefficients.

Figure 4-48. Recommended External Component

Connections

To other modules

Internal

External

KHZ32_XCI

KHZ32_XCO

Table 4-29. External Component Recommended Specifications

Component

Parameters

Values

Tolerance

Crystal

Resonance

32.768 kHz Parallel Mode

User-defined

Type

N-Cut or XY-bar

Serial Resistance

40 k

Ω

Max

Q Factor

35000

Min

Shunt Capacitance

2 pF

Max

Load Capacitance, CL

9-13 pF

Temperature Coefficient

User-defined

Resistor, R1

Resistor

20 M

Ω (Note 1)

Capacitor, C1

Capacitor

22 pF

Capacitor, C2

Capacitor

22 pF

Note 1. A single 20 M

Ω resistor may be difficult to acquire. Substituting two 10 MΩ resistors in series is acceptable.

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