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820E Chipset
R
Design Guide 89
Figure 54. CDC_DN_ENAB# Support Circuitry for Two-Codecs on Motherboard / Two-Codecs on
CNR
Codec A
RESET#
SDATA_IN
Codec C
RESET#
SDATA_IN
AC97_RESET#
Vcc
CDC_DN_ENAB#
CNR BoardMotherboard
R
A
10 kΩ
ΩΩ
Ω
R
B
1 κΩ
κΩ κΩ
κΩ
To General
Purpose Input
From AC '97
Controller
CNR Connector
To AC '97
Digital
Controller
SDATA_IN0
SDATA_IN1
Codec D
RESET#
SDATA_IN
Codec B
RESET#
SDATA_IN
Circuit Notes
1. While it is possible to disable down codecs, as shown above in Figure 53 and Figure 54, it is
recommended against for reasons cited in the ICHx/AC'97 White Paper, including avoidance of
shipping redundant and/or non-functional audio jacks.
2. All CNR designs include resistor R
B
. The value of R
B
is either 1 kΩ or 100 kΩ, depending on the
intended functionality of the CNR (whether or not it intends to be the primary/controlling codec).
3. Any CNR with two codecs must implement R
B
with value 1 kΩ. If there is one codec, use a
100 kΩ pull-up resistor. A CNR with zero codecs must not stuff R
B
. If implemented, R
B
must be
connected to the same power well as the codec so that it is valid whenever the codec has power.
4. A motherboard with one or more codecs down must implement R
A
with a value of 10 kΩ.
5. The CDC_DN_ENAB# signal must be run to a GPI so that the BIOS can sense the state of the
signal. CDC_DN_ENAB# is required to be connected to a GPI; a connection to a GPIO is
strongly recommended for testing purposes.
Table 18. Signal Descriptions
CDC_DN_ENAB# When low, indicates that the codec on the motherboard is enabled and
primary on the AC’97 Interface. When high, indicates that the motherboard
codec(s) must be removed from the AC’97 Interface (held in reset), because
the CNR codec(s) will be the primary device(s) on the AC’97 Interface.
AC97_RESET# Reset signal from the AC’97 Digital Controller (ICH2).
SDATA_INn AC’97 serial data from an AC’97-compliant codec to an AC’97-compliant
controller (i.e., the ICH2).