Input/Output Interfaces

5.8.1 AC97 Audio Controller

The AC97 Audio Controller is a PCI device that is integrated into the 82801 ICH component and supports the following functions:

■Read/write access to audio codec registers

■16-bit stereo PCM output @ up to 48 KHz sampling

■16-bit stereo PCM input @ up to 48 KHz sampling

■Acoustic echo correction for microphone

■AC'97 Link Bus

■ACPI power management

5.8.2AC97 Link Bus

The audio controller and the audio codec communicate over a five-signal AC97 Link Bus (Figure 5-12). The AC97 Link Bus includes two serial data lines (SD OUT/SD IN) that transfer control and PCM audio data serially to and from the audio codec using a time-division multiplexed (TDM) protocol. The data lines are qualified by a 12.288 MHz BIT_CLK signal driven by the audio codec. Data is transferred in frames synchronized by the 48-KHz SYNC signal, which is derived from the clock signal and driven by the audio controller. The SYNC signal is high during the frame's tag phase then falls during T17 and remains low during the data phase. A frame consists of one 16-bit tag slot followed by twelve 20-bit data slots. When asserted (typically during a power cycle), the RESET- signal (not shown) will reset all audio registers to their default values.

T1

T2

T18

T19

T38

T39

T58

BIT_CLK (12.288 MHz)

SYNC

 

 

 

 

 

 

(48 KHz)

 

 

 

 

 

 

SD OUT

Codec Bit 15 Bit 14

Bit 0

Bit 19 Bit 18

Bit 0

Bit 19 Bit 18

Bit 0 Bit 19

or SD IN

Ready

 

 

 

 

 

 

 

 

 

 

 

 

Slot 0 (Tag)

Slot 1 (Data)

Slot 2 (Data)

Slot

Description

 

 

 

 

 

0

Bit 15: Frame valid bit

 

 

 

 

 

 

Bits 14-3: Slots 1-12 valid bits

 

 

 

 

 

Bits 2-0: Codec ID

 

 

 

 

 

1

Command address: Bit 19, R/W; Bits 18..12, reg. Index; Bits 11..0, reserved.

2

Command data

 

 

 

 

 

3

Bits 19-4: PCM audio data, left channel (SD OUT, playback; SD IN, record)

 

Bits 3-0 all zeros

 

 

 

 

 

4

Bits 19-4: PCM audio data, right channel (SD OUT, playback; SD IN, record)

 

Bits 3-0 all zeros

 

 

 

 

 

5

Modem codec data (not used in this system)

 

 

 

6-11

Reserved

 

 

 

 

 

12

I/O control

 

 

 

 

 

Figure 5-12. AC97 Link Bus Protocol

Technical Reference Guide

361834-002

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