WM8321 datasheet(48/253 Pages) WOLFSON

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WM8321

Production Data

PD, February 2012, Rev 4.0

13.3 CRYSTAL OSCILLATOR CONNECTIONS

The crystal oscillator generates a 32.768kHz reference clock, which is used to provide reference

clock for the Real Time Clock (RTC) in the WM8321. The oscillator requires an external crystal on the

XTI and XTO pins, as well as two capacitors, connected as shown in Figure 17.

XTO

WM8321

Crystal Oscillator

XTI

32kHz

Cxt

Lxt

Rxt

Crystal Model

XOSCGND

Figure 17 Crystal Oscillator

A suitable crystal oscillator should be selected in accordance with the following requirements:

PARAMETER

MIN

MAX

UNITS

Nominal frequency

32.768

kHz

Series resistance



Maximum driving level

0.5

W

Table 18 Selection of Crystal Oscillator Component

The load capacitors C1 and C2 should be selected according to the recommended load capacitance,

CL of the crystal, which is given by the following equation:

Assuming C1 = C2 and CSTRAY = 2.75pF (typical pad i/o capacitance), then:

C1 = C2 = 2 x (CL - 2.75pF).

For example, if the crystal has a load capacitance CL = 9pF, then C1 = C2 = 12.5pF.

If a suitable 32.768kHz clock is already present elsewhere in the system, it is possible for the

WM8321 to use that external clock instead. The external clock should be applied to pin XTI, and the

XTO pin left floating in this case.

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