WM9707

Advanced Information

WOLFSON MICROELECTRONICS LTD

AI Rev 2.2 January 2001

14

SYNC

BIT_CLK

SDATA_OUT

VALID

FRAME

SLOT(1)

SLOT(2)

SLOT(12)

’0’

(ID1)

(ID0)

19

0

19

0

19

0

19

0

TAG PHASE

DATA PHASE

20.8

µS (48kHz)

12.288MHz

81.4nS

END OF PREVIOUS

AUDIO FRAME

TIME SLOT ’VALID’ BITS

(’1’ = TIME SLOT CONTAINS

VALID PCM DATA)

SLOT (1)

SLOT (2)

SLOT (3)

SLOT (12)

Figure 11 AC-link Audio Output Frame

The datastreams currently defined by the AC’97 specification include:

PCM playback - 2 output slots

2-channel composite PCM output stream

PCM record data - 2 input slots

2-channel composite PCM input stream

Control - 2 output slots

Control Register write port

Status - 2 input slots

Control Register read port

Optional modem line codec output -

1 output slot

Modem line codec DAC input stream

Optional modem line codec input –

1 input slot

Modem line codec ADC output stream

Optional dedicated microphone input -

1 input slot

Dedicated microphone input stream in support

of stereo AEC and/or other voice applications.

Synchronisation of all AC-link data transactions is signalled by the WM9707 controller. The WM9707

drives the serial bit clock onto AC-link, which the AC’97 controller then qualifies with a

synchronisation signal to construct audio frames.

SYNC, fixed at 48kHz, is derived by dividing down the serial clock (BIT_CLK). BIT_CLK, fixed at

12.288MHz, provides the necessary clocking granularity to support 12, 20-bit outgoing and incoming

time slots. AC-link serial data is transitioned on each rising edge of BIT_CLK. The receiver of AC-link

data, (WM9707 for outgoing data and AC’97 controller for incoming data), samples each serial bit on

the falling edges of BIT_CLK.

The AC-link protocol provides for a special 16-bit time slot (slot 0) wherein each bit conveys a valid

tag for its corresponding time slot within the current audio frame. A 1 in a given bit position of slot 0

indicates that the corresponding time slot within the current audio frame has been assigned to a data

stream, and contains valid data. If a slot is tagged invalid, it is the responsibility of the source of the

data, (the WM9707 for the input stream, AC’97 controller for the output stream), to stuff all bit

positions with 0s during that slot’s active time.

SYNC remains high for a total duration of 16 BIT_CLKs at the beginning of each audio frame.

The portion of the audio frame where SYNC is high is defined as the Tag Phase. The remainder of

the audio frame where SYNC is low is defined as the Data Phase. Additionally, for power savings, all

clock, sync, and data signals can be halted. This requires that the WM9707 be implemented as a

static design to allow its register contents to remain intact when entering a power savings mode.