WM8711SEDSSLASHR datasheet(23/47 Pages) WOLFSON

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Production Data

WM8711 / WM8711L

PD Rev 4.4 April 2007

CLOCKING SCHEMES

In a typical digital audio system there is only one central clock source producing a reference clock to

which all audio data processing is synchronised. This clock is often referred to as the audio system’s

Master Clock. To allow WM8711/L to be used in a centrally clocked system, the WM8711/L is

capable of either generating this system clock itself or receiving it from an external source as will be

discussed.

For applications where it is desirable that the WM8711/L is the system clock source, then clock

generation is achieved through the use of a suitable crystal connected between the XTI/MCLK input

and XTO output pins (see CRYSTAL OSCILLATOR section).

For applications where a component other than the WM8711/L will generate the reference clock, the

external system Master Clock can be applied directly through the XTI/MCLK input pin with no

software configuration necessary. Note that in this situation, the oscillator circuit of the WM8711/L can

be safely powered down to conserve power (see POWER DOWN section)

CORE CLOCK

The WM8711/L DSP core can be clocked either by MCLK or MCLK divided by 2. This is controlled by

software as shown in Table 7 below.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001000

Sampling

Control

CLKIDIV2

Core Clock divider select

1 = Core Clock is MCLK divides by 2

0 = Core Clock is MCLK

Table 7 Software Control of Core Clock

Having a programmable MCLK divider allows the device to be used in applications where higher

frequency master Clocks are available. For example the device can support 512fs master clocks

whilst fundamentally operating in a 256fs mode.

CRYSTAL OSCILLATOR

The WM8711/L includes a crystal oscillator circuit that allows the audio system’s reference clock to

be generated on the device. This is available to the rest of the audio system in buffered form on

CLKOUT. The crystal oscillator is a low radiation type, designed for low EMI. A typical application

circuit is shown Figure 12.

XTI/MCLK

XTO

DGND

Figure 12 Crystal Oscillator Application Circuit

The WM8711/L crystal oscillator provides an extremely low jitter clock source. Low jitter clocks are a

requirement for high quality audio DACs, regardless of the converter architecture. The WM8711/L

architecture is less susceptible than most converter techniques but still requires clocks with less than

approximately 1ns of jitter to maintain performance. In applications where there is more than one

source for the master clock, it is recommended that the clock is generated by the WM8711/L to

minimise such problems.

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