AC’97 Controller Unit

Figure 13-3. AC-link Audio Output Frame

 

 

Tag Phase

 

 

 

 

20.8uS

Data Phase

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(48 KHz)

 

 

 

 

 

SYNC

12.288 MHz

 

 

 

 

 

 

 

 

 

 

 

 

81.4 nS

 

 

 

 

 

 

 

 

 

 

BIT_CLK

 

 

 

 

 

 

 

 

 

 

 

 

SDATA_OUT

Valid

slot(1) slot(2)

slot(12) "0"

codec ID codec ID

19

0

19

0

19

0

19

0

Frame

 

 

 

 

 

 

 

 

 

 

 

 

End of previous

 

Time Slot "Valid"

 

Slot 1

 

 

Slot 2

 

Slot 3

 

Slot 12

Audio Frame

 

 

 

 

 

 

 

 

 

Bits

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

A new audio output frame begins with a low-to-high SYNC transition synchronous to BITCLK’s rising edge. BITCLK’s falling edge immediately follows and AC’97 samples SYNC’s assertion. BITCLK’s falling edge marks the instance that AC-link’s sides are each aware that a new audio frame has started. On BITCLK’s next rising edge, the ACUNIT transitions SDATA_OUT into the slot 0’s first bit position (valid frame bit). Each new bit position is presented to AC-link on a BITCLK rising edge and then sampled by AC’97 on the following BITCLK falling edge. This sequence ensures that data transitions and subsequent sample points for both incoming and outgoing data streams are time aligned.

Figure 13-4. Start of Audio Output Frame

 

AC '97 samples SYNC assertion here

SYNC

AC '97 samples first SDATA_OUT bit of frame here

BIT_CLK

 

 

SDATA_OUT

Valid

slot(1) slot(2)

Frame

 

 

End of previous

Audio Frame

The SDATA_OUT composite stream is MSB-justified (MSB first). The ACUNIT fills all non-valid slot bit positions with zeroes. If fewer than 20 valid bits exist in an assigned valid time slot, the ACUNIT stuffs all trailing non-valid bit positions of the 20-bit slot with zeroes.

For example, if a 16-bit sample stream is being played to an AC’97 DAC, the first 16 bit positions are presented to the DAC MSB-justified. They are followed by the next four bit positions that the ACUNIT stuffs with zeroes. This process ensures that the least significant bits do not introduce any DC biasing, regardless of the implemented DAC’s resolution (16-, 18-, or 20-bit).

Note: When the ACUNIT transmits mono audio sample streams, software must ensure that the left and right sample stream time slots are filled with identical data.

Intel® PXA255 Processor Developer’s Manual

13-5

Page 457
Image 457
Intel PXA255 manual AC-link Audio Output Frame