Intel® PXA26x Processor Family Developer’s Manual 13-5
AC97 Controller Unit
A new audio output frame begins with a low-to-high SYNC tr ansition synchronous to BITCLK’s
rising edge. BITCLK’s falling edge immediately follows and AC97 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 ne w bit position is presented to AC-link on a
BITCLK rising edge and then sampled by AC97 on the following BIT CL K falling edge. This
sequence ensures that data transitions and subsequent sam ple points for both incoming and
outgoing data streams are time aligned.
The SDATA_OU T composite str eam is MSB-jus tified (MS B first). The ACUNIT fi lls 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 AC97 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 ensur es th at t he l east si gni ficant bit s do no t intr oduce a ny
DC biasing, regardless of the implemented DAC’ s resolution (16-, 18-, or 20-bit).
Figure 13-3. AC-link Audio Output Frame
SYNC
BIT_CLK
SDATA_OUT
slot(1)
Time Slot "Valid"
Bits
20.8uS
(48 KHz)
Slot 1 Slot 2
0 19 0 19 0 19 0
Slot 3 Slot 12
81.4 nS
12.288 MHz
slot(2)
codec IDcodec ID
"0"slot(12)
("1" = time slot contains valid PCM data)
19
Valid
Frame
End of previous
Audio Frame
Tag Phase Data Phase
Figure 13-4. Start of Audio Output Frame
SYNC
SDATA_OUT
slot(1) slot(2)
Valid
Frame
End of previous
Audio Frame
AC '97 samples SYNC assertion here
AC '97 samples first SDATA_OUT bit of frame here
BIT_CLK