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Intel® ICH5 AC ’97 Controller Theory of Operation
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28 AC ’97 Programmer’s Reference Manual
Table 12. SDM Register Description
Bit Type Reset Description
7:6 RW 00
PCM In 2, Microphone In 2 Data In Line (D21L): When the SE bit is set, these bits
indicate which SDATA_IN line should be used by the hardware for decoding the
input slots for PCM In 2 and Microphone In 2. When the SE bit is cleared, the value
of these bits is irrelevant, and PCM In 2 and Mic In 2 DMA engines are not
available.
00 SDATA_IN0
01 SDATA_IN1
10 SDATA_IN2
11 Reserved
5:4 RW 00
PCM In 1, Microphone In 1 Data In Line (DI1L): When the SE bit is set, these bits
indicates which SDATA_IN line should be used by the hardware for decoding the
input slots for PCM In 1 and Microphone In 1. When the SE bit is cleared, the value
of these bits are irrelevant, and the PCM In 1 and Mic In 1 engines use the OR’d
SDATA_IN lines.
00 SDATA_IN0
01 SDATA_IN1
10 SDATA_IN2
11 Reserved
3 RW 0
Steer Enable (SE): When set, the SDATA_IN lines are treated separately and not
OR’d together before being sent to the DMA engines. When cleared, the
SDATA_IN lines are OR’d together, and the “Microphone In 2” and “PCM In 2”
DMA engines are not available.
2 RO 0 Reserved
1:0 RO 00
Last Codec Read Data Input (LDI): When a codec register is read, this indicates
which SDATA_IN the read data returned on. Software can use this to determine
how the codecs are mapped. The values are:
00 SDATA_IN0
01 SDATA_IN1
10 SDATA_IN2
11 Reserved
3.5.1 Codec Register Shadowing
Codec register reads are presented in the AC-link in the next available frame after the controller
receives the I/O transaction. Data will be returned to the controller pending codec availability. To
avoid longer latencies than necessary, the codec must return data in the next available frame.
Multiple frame transactions impose large system latencies, to the detriment of system performance.
Even when data is returned in the frame immediately after the read request is presented in the AC-
link, the minimum latency is still on the order of 40 µs. To minimize the effect on the system
caused by long latencies in the AC-link, software drivers must maintain a copy of the codec
register in memory (shadow) and use this data instead of accessing the codec.