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10.3.2
AC'97 Signal Quality Requirements
In a lightly loaded system (e.g., single codec down), AC'97 signal integrity should be evaluated to
confirm that the signal quality on the link is acceptable to the codec used in the design. A series
resistor at the driver and a capacitor at the codec can be implemented to compensate for any signal
integrity issues. The values used will be design dependent and should be verified for correct
timings. The ICH AC-link output buffers are designed to meet the AC'97 2.1 specification, with
the specified load of 50 pF.
10.3.3
Motherboard Implementation
The following design considerations are provided for the implementation of an ICH platform using
AC'97. These design guidelines have been developed to ensure maximum flexibility for board
designers, while reducing the risk of board-related issues. These recommendations are not the only
implementation or a complete checklist, but they are based on the ICH platform.
Codec Implementation
Any valid combination of codecs may be implemented on the motherboard and on the
riser. For ease of homologation, it is recommended that a modem codec be implemented
on a CNR module. However, nothing precludes a modem codec on the motherboard.
Only one primary codec may be present on the link. A maximum of two codecs can be
supported in an ICH platform.
Components (e.g., FET switches, buffers or logic states) should not be implemented on
the AC-link signals, except for AC_RST#. Doing so would potentially interfere with
timing margins and signal integrity.
The ICH supports wake-on-ring from S1–S4 states via the AC'97 link. The codec asserts
SDATAIN to wake the system. To provide wake capability and/or caller ID, standby
power must be provided to the modem codec. If no codec is attached to the link, internal
pull-downs will prevent the inputs from floating, so external resistors are not required.
The ICH does not wake from the S5 state via the AC'97 link.
The SDATAIN[0:1] pins should not be left in a floating state if the pins are not connected
and the AC-link is active. Rather, they should be pulled to ground through a weak
(approximately 10 k ) pull-down resistor. If the AC-link is disabled (by setting the shut-
off bit to 1), then the ICH's internal pull-down resistors are enabled, so there is no need
for external pull-down resistors. However, if the AC-link is to be active, then there should
be pull-down resistors on any SDATAIN signal that might not be connected to a codec.
For example, if a dedicated audio codec is on the motherboard and cannot be disabled via
a hardware jumper or stuffing option, then its SDATAIN signal does not need a pull-
down resistor. However, if the SDATAIN signal has no codec connected or is connected
to an on-board codec that can be hardware-disabled, then the signal should have an
external pull-down resistor to ground.
The ICH provides internal weak pull-downs. Therefore, the motherboard does not need to
provide discrete pull-down resistors.
PC_BEEP should be routed through the audio codec. Care should be taken to avoid the
introduction of a pop when powering the mixer up or down.
®
Intel
815 Chipset Platform Design Guide
I/O Subsystem
121
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