STAC9708 datasheet(15/56 Pages) ETC

15 / 56 page

SigmaTel, Inc.

Preliminary

STAC9708/11

10/02/98

Figure 5. AC'97 Standard Bi-directional Audio Frame

3.1.1 AC-Link Audio Output Frame (SDATA_OUT)

The audio output frame data streams correspond to the multiplexed bundles of all digital output data

targeting the STAC9708/11 DAC inputs, and control registers. Each audio output frame supports up to

12 20-bit outgoing data time slots. Slot 0 is a special reserved time slot containing 16 bits that are used

for AC-Link protocol infrastructure.

Within slot 0, the first bit is a global bit (SDATA_OUT slot 0, bit 15) which flags the validity for the

entire audio frame. If the “Valid Frame” bit is a 1, this indicates that the current audio frame contains

at least one slot time of valid data. The next 12 bit positions sampled by the STAC9708/11 indicate

which of the corresponding 12 times slots contain valid data. In this way data streams of differing

sample rates can be transmitted across AC-Link at its fixed 48kHz audio frame rate. The following

diagram illustrates the time slot based AC-Link protocol.

Figure 6. AC-Link Audio Output Frame

SYNC

BIT_CLK

SDATA_OUT

valid

Frame

slot1

slot2

End of previous audio frame

slot(12) "0"

"0"

Data Phase

20.8 uS (48 kHZ)

Tag Phase

12.288 MHz

Time Slot "Valid" Bits

Slot 1

Slot 2

Slot 3

Slot 12

("1" = time slot contains valid PCM data)

CID1

CID0

A new audio output frame begins with a low to high transition of SYNC. SYNC is synchronous to the

rising edge of BIT_CLK. On the immediately following falling edge of BIT_CLK, the STAC9708/11

samples the assertion of SYNC. This following edge marks the time when both sides of AC-Link are

aware of the start of a new audio frame. On the next rising edge of BIT_CLK, the AC'97 controller

transitions SDATA_OUT into the first bit position of slot 0 (Valid Frame bit). Each new bit position is

presented to AC-Link on a rising edge of BIT_CLK, and subsequently sampled by the STAC9708/11 on

OUTGOING STREAMS

INCOMING STREAMS

SYNC

TAG PHASE

TAG

CMD

ADR

CMD

DATA

PCM

LEFT

PCM

RSVD

PCM

LEFT

PCM

STATUS

DATA

STATUS

ADR

TAG

DATA PHASE

PCM

CTR

PCM

LSURR

PCM

RSURR

PCM

LFE

PCM

LALT

PCM

RALT

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