WM9708

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 April 2001

10

SYSTEM INFORMATION

WM9708

AC’97

DIGITAL

CONTROLLER

RESETB

BITCLK

SYNC

SDATAIN

SDATAOUT

PCBEEP

CD, LINEINL/R

MONO_OUT

LINEOUTL/R

MIC1

26

25

12

16

11

7

10

9

6

24

Figure 7 Revision 2.1 Compliant 2-Channel Codec

DEVICE DESCRIPTION

INTRODUCTION

The WM9708 comprises a stereo 18-bit Codec, (that is, 2 ADCs and 2 DACs) a comprehensive

analogue mixer with 2 sets of stereo inputs, phone, microphone, and PC-beep inputs. Additionally,

on-chip reference generation circuits generate the necessary bias voltages for the device, and a bi-

directional serial interface allows transfer of control data and DAC and ADC words to and from the

AC’97 controller. The WM9708 supports 18-bit resolution within the DAC and ADC functions, but the

AC’97 serial interface specification allows any word length up to 20-bits to be written to, or read from,

the AC’97 Codec. These words are MSB justified, and any LSBs not used will simply default to 0.

Normally it is anticipated that 16-bit words will be used in most PC type systems. Therefore, for the

DAC, 16-bit words will be downloaded into the Codec from the controller, along with padding of 0s to

make the 16-bit word up to 20-bit length. In this case, the WM9708 will process the 16-bit word along

with 0 padding bits in the 2 LSB locations (to make 18-bit). At the ADC output, WM9708 will provide

an 18-bit word, again with 0s in the two LSB locations (20-bit). The AC’97 controller will then ignore

the 4 LSBs of the 20-bit word. When the WM9708 is interrogated at Register 00h, it responds

indicating it is an 18-bit device.

The WM9708 has the ADC and DAC functions implemented using oversampled, or sigma-delta

converters, and uses on-chip digital filters to convert these 1-bit signals to and from the 48ks/s 16/18-

bit PCM words that the AC’97 controller requires. The digital parts of the device are powered

separately from the analogue to optimise performance, and 3.3V digital and 5V analogue supplies

may be used on the same device to further optimise performance. Digital I/Os are 5V tolerant when

the analogue supplies are 5V, so the WM9708 may be connected to a controller running on 5V

supplies, but use 3.3V for the digital section of WM9708. WM9708 is also capable of operating with a

3.3V supply only (digital and analogue).

An internally generated midrail reference is provided at pin CAP which is used as the chip reference.

This pin should be heavily decoupled. Refer to Figure 15 for more details.

The WM9708 is not limited to PC-only applications. The ability to power down sections of the device

selectively, and the option to choose alternative master clock, and hence sample rates, means that

many alternative applications in areas such as telecoms, may be anticipated.

Additional features added to the Intel AC’97 2.1 specification, such as the internal connection of PC-

beep to the outputs in the case where the device is reset, are supported, along with optional features

such as variable sample rate support.