
Functional Description
5-64
82801AA and 82801AB Datasheet
Since some SMBus devices are not powered during some low power states, a separate SMBus
interface is provided to connect the ICH to the Alert On LAN network controller. The signals that
comprise this interface (referred to as ALERTCLK and ALERTDATA) are multiplexed with
GPIO[27:28]. At reset, these signals default to ALERTCLK and ALERTDATA. Note that Alert on
LAN messages are broadcast
only
over this separate interface, and the ICH looks for an ACK from
the LAN controller on the ALERTDATA signal.
If a unified SMBus architecture is desired, the SMBDATA and ALERTDATA (and SMBCLK and
ALERTCLK) signals can be “ORed” together since they are both open drain driven signals.
However, the ICH does not check to see if the SMBus is busy before broadcasting an Alert on
LAN message using the ALERTCLK and ALERTDATA signals. If the two interfaces are tied
together, it is important that there be no SMBus masters other than the ICH in the system. It is also
necessary to provide any necessary logic to isolate devices powered by standby from devices
powered by core power.
If the system is in a G1 (S1-S4) state, the ICH sends a heartbeat message every 30-32 seconds. If
an event occurs prior to the system being shutdown, the ICH immediately sends an event message
with the next incremented sequence number. After the event message, the ICH resumes sending
heartbeat messages.
Note that, for all message fields more than 1 bit, the most significant bit (MSB) is tranferred first.
The most significant bit is on the left. For example, for 100000 the “1” is the MSB.
All Alert On LAN messages are broadcast over the ALERTCLK and ALERTDATA pins that are
multiplexed with GPIO[27] and GPIO[28]. There are two types of Alert On LAN messages;
heartbeat messages and event messages. Heartbeat messages occur every 32 seconds. When
coming out of G3, the first heartbeat message could occur anytime within the first 32 seconds. The
second heartbeat message occurs 32 seconds after the first. However, event messages occur right
after certain events occur. One such event is the act of entering into the pre-boot state.
Coming out of a G3 state ALERTCLK/GPIO[27] and ALERTDATA/GPIO[28] default to
ALERTCLK and ALERTDATA. These signals can be programmed as GPIOs. However, before
the BIOS has a chance to program these signals to GPIOs, the pre-boot event message and
possibly a heartbeat message could be driven onto ALERTCLK and ALERTDATA.
If ALERTCLK and ALERTDATA are left floating, the SMBus state machine could get stuck in a
busy state indefinitely when an Alert On LAN message goes out. To prevent this condition, the
ICH requires external pull-ups to 3.3V standby on ALERTCLK and ALERTDATA.
If ALERTCLK and ALERTDATA are to be used as GPIOs, they need to be pulled up to 3.3V
standby to avoid the SMBus from getting stuck. Also, when coming out of a G3 state, the devices
that are connected to the GPIOs must be able to withstand the ICH driving Alert On LAN
messages on these pins until the BIOS has a chance to program these pins as GPIOs. Examples of
devices that would not be affected by this behavior include LEDs.