WM9708

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.0 April 2001

18

SLOT 4: PCM RECORD RIGHT CHANNEL

Audio input frame slot 4 is the right channel output of the WM9708’s input Mux, post-ADC.

The WM9708’s ADCs can be implemented to support 16, 18, or 20-bit resolution.

The WM9708 ships out its ADC output data (MSB first), and stuffs any trailing non-valid bit positions

with 0s to fill out its 20-bit time slot.

SLOT 5: OPTIONAL MODEM LINE CODEC

Not supported by WM9708.

SLOT 6: OPTIONAL DEDICATED MICROPHONE RECORD DATA

Not supported by WM9708.

SLOTS 7 TO 12: RESERVED

Audio input frame slots 7 to 12 are reserved for future use and are always stuffed with 0s by the

WM9708.

AC-LINK LOW POWER MODE

The AC-link signals can be placed in a low power mode. When the WM9708’s Powerdown Register

26h, is programmed to the appropriate value, both BITCLK and SDATAIN will be brought to, and held

at a logic low voltage level.

BITCLK and SDATAIN are transitioned low immediately following the decode of the write to the

Powerdown Register 26h with PR4. When the AC’97 controller driver is at the point where it is ready

to program the AC-link into its low power mode, slots 1 and 2 are assumed to be the only valid

stream in the audio output frame. At this point in time it is strongly recommended that all sources of

audio input have also been neutralised.

The AC’97 controller should also drive SYNC and SDATAOUT low after programming the WM9708

to this low power, halted mode.

Once the WM9708 has been instructed to halt BITCLK, a special wake up protocol must be used to

bring the AC-link to the active mode since normal audio output and input frames can not be

communicated in the absence of BITCLK.

WAKING UP THE AC-LINK

There are 2 methods for bringing the AC-link out of a low power, halted mode. Regardless of the

method, it is the AC’97 controller that performs the wake up task.

AC-link protocol provides for a Cold WM9708 Reset, and a Warm WM9708 Reset.

The current Powerdown state would ultimately dictate which form of WM9708 reset is appropriate.

Unless a cold or register reset (a write to the Reset Register 00h) is performed, wherein the WM9708

registers are initialised to their default values, registers are required to keep state during all

Powerdown modes.

Once powered down, re-activation of the AC-link via re-assertion of the SYNC signal must not occur

for a minimum of 4 audio frame times following the frame in which the Powerdown was triggered.

When AC-link powers up it indicates readiness via the Codec Ready bit (input slot 0, bit 15).

COLD WM9708 RESET

A cold reset is achieved by asserting RESETB for the minimum specified time (1

µs). By driving

RESETB low, BITCLK, and SDATAOUT will be activated, or re-activated as the case may be, and all

the WM9708 control registers will be initialised to their default power on reset values.

RESETB is an asynchronous WM9708 input. See Figure 2.