PC87591L datasheet(357/437 Pages) NSC

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Host-Controlled Modules and Host Interface (Continued)

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Table 65. Crystal Oscillator Circuit Components

External Elements

Choose C1 and C2 capacitors (see Figure 113) 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 ca-

pacitance is caused by the chip package, board layout and socket (if any), and can vary from 0 to 8 pF. The rule of thumb

for choosing these capacitors is:

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

Oscillator Start-Up

The oscillator starts to generate 32.768 KHz pulses to the RTC after approximately t32KW from when VBAT is higher than

VBATMIN (2.4 V) or from when VCC is higher than VCCMIN (3.0V).

C1 can be trimmed to achieve precisely 32.768 KHz. To achieve a high time accuracy, use crystal and capacitors with low

tolerance and temperature coefficients.

6.2.4

External Oscillator

32.768 KHz can be applied from an external clock source, as shown in Figure 114.

Figure 114. External Oscillator Connections

Connections

Connect the clock to the 32KCLKIN pin, leaving the oscillator output, 32KX2, unconnected.

Component

Parameters

Values

Tolerance

Crystal

Resonance Frequency

32.768 KHz Parallel Mode

User deﬁned

Type

N-Cut or XY-bar

Serial Resistance

40 K

Ω

Max

Quality Factor, Q

35000

Min

Shunt Capacitance

2 pF

Max

Load Capacitance, CL

9-13 pF

Temperature Coefﬁcient

User deﬁned

Resistor R1 Resistance

20 M

Ω

Resistor R2 Resistance

121 K

Ω

Capacitor C1 Capacitance

10 pF

10%

Capacitor C2 Capacitance

12 pF

10%

32.768 KHz

Clock Generator

Internal

External

32KCLKIN /

32KX2

To other

modules

Battery

VBAT

OUT

POWER

CF = 0.1

µF

32KX1

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