SigmaTel, Inc.
Data Sheet
STAC9721
31
04/07/00
04/07/00
6. MULTIPLE CODEC SUPPORT
The STAC9721/23 provides support for the multi-codec option according to the Intel AC'97, rev 2.1
specification. By definition there can be only one Primary Codec (Codec ID 00) and up to three
Secondary Codecs (Codec IDs 01,10, and 11). The Codec ID functions as a chip select. Secondary
devices therefore have completely orthogonal register sets; each is individually accessible and they do not
share registers.
6.1 Primary/Secondary Codec Selection
In a multi-codec environment the codec ID is provided by external programming of pins 45 and 46 (CID0
and CID1). The CID pin electrical function is logically inverted from the codec ID designation. The
corresponding pin state and its associated codec ID are listed in the "Codec ID Selection" table. Also see
slot assignment discussion, “Multi-Channel Programming Register (Index 74)”.
Table 21. Codec ID Selection
CID1 State
CID0 State
Codec ID
Codec Status
EXTENDED AUDIO
ID, 28h
ID1, ID0
+5V or floating
+5V or floating
00
Primary
0, 0
+5V or floating
0V
01
Secondary
0, 1
0V
+5V or floating
10
Secondary
1, 0
0V
0V
11
Secondary
1, 1
6.1.1 Primary Codec Operation
As a Primary device the STAC9721/23 is completely compatible with existing AC'97
definitions and extensions. Primary Codec registers are accessed exactly as defined in
the AC'97 Component Specification and AC'97 Extensions. The STAC9721/23 operates
as Primary by default, and the external ID pins (47 and 48) have internal pull-ups so that
these pins may be left as no-connects for primary operation.
When used as the Primary Codec, the STAC9721/23 generates the master AC-Link
BIT_CLK for both the AC'97 Digital Controller and any Secondary Codecs. The
STAC9721/23 can support up to 4, 10 K
Ω 50 pF loads on the BIT_CLK. This is to
insure that up to 4 Codec implementations will not load down the clock output.
6.1.2 Secondary Codec Operation
When the STAC9721/23 is configured as a Secondary device the BIT_CLK pin is
configured as an input at power up. Using the BIT_CLK provided by the Primary Codec
insures that everything on the AC-Link will be synchronous. As a Secondary device it
can be defined as Codec ID 01, 10, or 11 in the two-bit field(s) of the Extended Audio
and/or Extended Modem ID Register(s).
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