Advanced Information

WM9708

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 April 2001

11

VARIABLE SAMPLE RATE SUPPORT

The DACs and ADCs on this device support all the recommended sample rates specified in the Intel

Revision 2.1 specification for audio rates. The default rate is 48ks/s. If alternative rates are selected

and variable rate audio is enabled (Register 2Ah, bit 0), the AC’97 interface continues to run at 48k

words per second, but data is transferred across the link in bursts such that the net sample rate

selected is achieved. It is up to the AC’97 Revision 2.1 compliant controller to ensure that data is

supplied to the AC link, and received from the AC link, at the appropriate rate.

The device supports on demand sampling. That is, when the DAC signal processing circuits need

another sample, a sample request is sent to the controller which must respond with a data sample in

the next frame it sends. For example, if a rate of 24ks/s is selected, on average the device will

request a sample from the controller every other frame, for each of the stereo DACs. Note that if an

unsupported rate is written to one of the rate registers, the rate will default to the nearest rate

supported. The Register will then respond when interrogated with the default sample rate.

The left and right channels of the ADCs and DACs always sample at the same rate.

AUDIO

SAMPLE RATE

CONTROL VALUE

D15-D0

8000

1F40

11025

2B11

16000

3E80

22050

5622

32000

7D00

44100

AC44

48000

BB80

Table 1 Variable Sample Rates Supported

GAIN CONTROL REGISTER LOCATION

PGA

CONTROL REGISTER

MUTE DEFAULT

DAC

18h

Muted (bit-15)

Mixer

72h

Not-muted (bit-15)

Volume

02h

Muted (15)

Table 2 Gain Control Register Location

MASTER SUPPORT

WM9708 supports operation as a master codec. Fundamentally, a device identified as a

master produces a BITCLK as an input.

CONTROL INTERFACE

A digital interface has been provided to control the WM9708 and transfer data to and from it. This

serial interface is compatible with the Intel AC’97 specification.

The main control interface functions are:

•

Control of analogue gain and signal paths through the mixer

•

Bi-directional transfer of ADC and DAC words to and from AC’97 controller

•

Selection of Powerdown down modes.