Intel VC820 - Desktop Board Motherboard Design Manual page 90

Layout/Routing Guidelines
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Codec Implementation
— The motherboard can implement any valid combination of codecs on the motherboard and
on the riser. For ease of homologation, it is recommended that a modem codec be
implemented on the AMR module; however, nothing precludes a modem codec on the
motherboard.
— Only one primary codec can be present on the link. A maximum of two present codecs can
be supported in an ICH0/ICH platform.
— If the motherboard implements an active primary codec on the motherboard and provides
an AMR connector, it must tie PRI_DN# to ground.
— The PRI_DN# pin is provided to indicate a primary codec is present on the motherboard.
Therefore, the AMR module and/or codec must provide a means to prevent contention
when this signal is asserted by the motherboard, without software intervention.
— Components (e.g., FET switches), buffers, or logic states should not be implemented on
the AC-link signals, except for AC_RST#. Doing so will potentially interfere with timing
margins and signal integrity.
— If the motherboard requires that an AMR module override a primary codec down, a means
of preventing contention on the AC-link must be provided for the onboard codec.
— The ICH0/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 pulldowns will prevent the inputs from floating, therefore external resistors are
not required. The ICH0/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—they should be pulled to ground through a weak
(approximately 10 KΩ) pull-down resistor. If the AC-link is disabled (by setting the shut-
off bit to 1), then the ICH0/ICH's internal pull-down resistors are enabled, and thus 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 has the potential of not
being 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. If however, the SDATAIN signal has
no codec connected, or is connected to an AMR slot, or is connected to an onboard codec
that can be hardware disabled, then the signal should have an external pull-down resistor
to ground.
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AMR Slot Special Connections
— AUDIO_MUTE#: No connect on the motherboard.
— AUDIO_PWRDN: No connect on the motherboard. Codecs on the AMR card should
implement a powerdown pin, per the AC'97 2.1 specification, to control the amplifier.
— MONO_PHONE: Connect top onboard audio codec if supported.
— MONO_OUT/PC_BEEP: Connect to SPKR output from the ICH0/ICH, or MONO_OUT
from onboard codec.
— PRIMARY_DN#: See discussion above.
— +5VDUAL/+5VSB: Connect to VCC5 core on the motherboard, unless adequate power
supply is available. An AMR card using this standby/dual supply should not prevent basic
operation if this pin is connected to core power.
— S/P-DIF_IN: Connect to ground on the motherboard.
— AC_SDATAIN[3:2]: No connect on the motherboard. The ICH0/ICH supports a
maximum of two codecs, which should be attached to SDATAIN[1:0].
— AC97_MSTRCLK: Connect to ground on the motherboard.
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The ICH0/ICH provides internal weak pulldowns. Therefore, the motherboard does not need
to provide discrete pulldown resistors.
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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.
2-64
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Intel 820 Chipset Design Guide