WM8311 datasheet(51/302 Pages) WOLFSON

51 / 302 page

Pre-Production

WM8311

PP, May 2012, Rev 3.1

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0 = RTC unclocked in BACKUP

1 = RTC maintained in BACKUP

(Note that XTAL_ENA must also

be set if the RTC is to be

maintained in BACKUP.)

Table 16 Clocking Control

13.2 CRYSTAL OSCILLATOR

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

clock for the Real Time Clock (RTC) in the WM8311. It may also be used as a reference input to the

FLL, for the purpose of generating other clocks. The oscillator requires an external crystal on the XTI

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

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 17 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

WM8311 to use that external clock instead. The external clock should be applied to pin XTI, and the

XTO pin left floating in this case.

WM8311 Datasheet(PDF) 51 Page - Wolfson Microelectronics plc

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