WM8731 datasheet(30/56 Pages) WOLFSON

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WM8731

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 February 2001

The side tone mode and attenuation is selected under software control using the SIDETONE bit as

shown in Table 10. In true side tone the output from the DAC (DACSEL) and line inputs (BYPASS)

should be deselected from the line output block. However, this can also be used to sum the DAC

output, line inputs and microphone inputs together.

The microphone boost gain control and

headphone output volume control and mutes are still operational in side tone mode. The maximum

signal at any point in the side tone path must be no greater than 1.0V rms at VDD = 3.3V, to avoid

distortion. This amplitude tracks linearly with AVDD.

DEVICE OPERATION

DEVICE RESETTING

The WM8731 contains a power on reset circuit that resets the internal state of the device to a known

condition. The power on reset is applied as DCVDD powers on and released only after the voltage

level of DCVDD crosses a minimum turn off threshold. If DCVDD later falls below a minimum turn on

threshold voltage then the power on reset is re-applied. The threshold voltages and associated

hysteresis are shown in the Electrical Characteristics table.

The user also has the ability to reset the device to a known state under software control as shown in

the table below.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001111

Reset Register

8:0

RESET

not reset

Reset Register

Writing 00000000 to register resets

device

Table 11 Software Control of Reset

When using the software reset. In 3-wire mode the reset is applied on the rising edge of CSB and

released on the next rising edge of SCLK. In 2-wire mode the reset is applied for the duration of the

ACK signal (approximately 1 SCLK period, refer to Figure 32).

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 WM8731 to be used in a centrally clocked system, the WM8731 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 WM8731 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 WM8731 will generate the reference clock, the

external system 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 WM8731 can be safely

powered down to conserve power (see POWER DOWN section).

CORE CLOCK

The WM8731 DSP core can be clocked either by MCLK or MCLK divided by 2. This is controlled by

software as shown in Table 12 below.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001000

Sampling

Control

CLKIDIV2

Core Clock divider select

1 = Core Clock is MCLK divided by 2

0 = Core Clock is MCLK

Table 12 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.

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