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Intel® ICH5 AC ’97 Controller Theory of Operation
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26 AC ’97 Programmer’s Reference Manual
3.4.3 FIFO Organization
The ICH5 AC ’97 controller supports 16-bit samples on all channels except PCM Out, which also
supports 20-bit samples.
Data will be written to the FIFO in sample pairs following the order of valid slots in a channel. For
example, for audio PCM In, the controller will check the first valid slot and add it to the FIFO first
entry as a word (16 bits). The next valid slot will be added as the second word entry in the FIFO to
create the PCM stereo sample pair. This behavior works under the assumption that the first valid
slot will be always the Left channel (slot 3) followed by Right channel in slot 4 in the same or
subsequent frame. If the codec transmits data repeating the slot, it will cause the controller to
misplace the sample in the FIFO. Codecs compatible with the ICH5 AC ’97 implementation
should always maintain the indicated order, and never use the same slot twice to transmit samples
to the controller. The figures below present some ICH5 compatible and incompatible
implementations using as a reference a two-channel implementation.
Figure 5. Compatible Implementation with Left and Right Sample Pair in Slot 3/4 Every Frame
Frame n
Frame n + 1
Frame n + 2
Frame n + 3
X
CMD
DATA
MDM
CDC
RSVD RSVD RSVD RSVD RSVD RSVD
I/O
Control
CMD
ADR
TAG X
X
CMD
DATA
MDM
CDC
MIC RSVD RSVD RSVD RSVD RSVD
I/O
Status
Status
Data
Status
ADR
TAG X
X
CMD
DATA
MDM
CDC
RSVD RSVD RSVD RSVD RSVD RSVD
I/O
Control
CMD
Data
CMD
ADR
TAG X
X
CMD
DATA
MDM
CDC
MIC RSVD RSVD RSVD RSVD RSVD
I/O
Status
Status
Data
Status
ADR
TAG X
4
CMD
DATA
5 6 7 8 9 10 11 12210 3
SYNC
Slot #
Figure 6. Compatible Implementation with Sample Rate Conversion Slots 3 and 4 Alternating
over Next Frame
Frame n
Frame n + 1
Frame n + 2
Frame n + 3
CMD
DATA
MDM
CDC
RSVD RSVD RSVD RSVD RSVD RSVD
I/O
Control
CMD
ADR
TAG X
X
CMD
DATA
MDM
CDC
MIC RSVD RSVD RSVD RSVD RSVD
I/O
Status
Status
Data
Status
ADR
TAG
CMD
DATA
MDM
CDC
RSVD RSVD RSVD RSVD RSVD RSVD
I/O
Control
CMD
Data
CMD
ADR
TAG X
X
CMD
DATA
MDM
CDC
MIC RSVD RSVD RSVD RSVD RSVD
I/O
Status
Status
Data
Status
ADR
TAG
4
CMD
DATA
5 6 7 8 9 10 11 12210 3
SYNC
Slot #