STAC9704 datasheet(14/49 Pages) HYNIX

14 / 49 page

SigmaTel, Inc.

STAC9704/7

10/02/98

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 STAC9704/7 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 STAC9704/7 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)

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 STAC9704/7

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 STAC9704/7 on

the following falling edge of BIT_CLK. This sequence ensures that data transitions, and subsequent

sample points for both incoming and outgoing data streams are time aligned.

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