STAC9752/9753
TWO-CHANNEL, 20-BIT, AC’97 2.3 CODECS WITH HEADPHONE DRIVE, SPDIF OUTPUT MICROPHONE & JACK SENSING
IDT™
30 STAC9752/9753
REV 3.3 1206
TWO-CHANNEL, 20-BIT, AC’97 2.3 CODECS WITH HEADPHONE DRIVE, SPDIF OUTPUT MICROPHONE & JACK SENSING
5.2. AC-Link Serial Interface Protocol
The AC‘97 Controller signals synchronization of all AC-Link data transactions. The AC‘97 CODEC,
Controller, or external clock source drives the serial bit clock (BIT_CLK) onto AC-Link, which the
AC‘97 Controller then qualifies with a synchronization signal (SYNC) to construct audio frames.
SYNC, fixed at 48 KHz, is derived by dividing down the serial bit clock (BIT_CLK). BIT_CLK, fixed at
12.288 MHz, 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 (CODEC for outgoing data and 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 (AC‘97 CODEC for the input stream, AC‘97 Controller for the output stream), to stuff all bit posi-
tions with 0 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 an
AC‘97 CODEC be implemented as a static design to allow its register contents to remain intact when
entering a power savings mode.
5.2.1. AC-Link Variable Sample Rate Operation
The AC-Link serial interconnect defines a digital data and control pipe between the Controller and
the CODEC. The AC-Link supports 12 20-bit slots at 48 KHz on SDATA_IN and SDATA_OUT. The
time division multiplexed (TDM) “slot-based” architecture supports a per-slot valid tag infrastructure
that the source of each slot’s data sets or clears to indicate the validity of the slot data within the cur-
rent audio frame. This tag infrastructure can be used to support transfers between Controller and
CODEC at any sample rate.
5.2.2. Variable Sample Rate Signaling Protocol
AC-Link’s tag infrastructure imposes FIFO requirements on both sides of the AC-Link. For example,
in passing a 44.1 KHz stream across the AC-Link, for every 480 audio output frames that are sent
across, 441 of them must contain valid sample data. Does the AC‘97 Digital Controller pass all 441
PCM samples followed by 39 invalid slots? Or does the AC‘97 Digital Controller evenly interleave
Table 5. The AC-Link input slots (transmitted from the CODEC)
Slot Name Description
0 SDATA_IN TAG MSBs indicate which slots contain valid data
1 STATUS ADDR read port MSBs echo register address; LSBs indicate which slots request data
2 STATUS DATA read port 16-bit command register read data
3, 4 PCM L&R ADC record 20-bit PCM data from Left and Right inputs
5 Modem Line 1 ADC 16-bit modem data from modem Line1 input
6-11 PCM ADC Record 20-bit PCM data - Alternative Slots for Input
12 GPIO Status GPIO read port and interrupt status
