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By default, the functionality and input buffer associated with ETS# are disabled. Also, the GMCH can
be programmed to send an SERR, SCI, or SMI message to the ICH4-M upon the assertion of this signal.
External thermal sensors that are suitable for the purpose described above would need to have a small
form factor and be able to accurately monitor the ambient temperature in the vicinity of the DDR system
memory.
Intel is currently in the process of enabling this feature on the Intel 855GM/GME chipset GMCH and is
actively engaging with thermal sensor vendors to ensure compatibility and suitability of vendors'
products with the ETS# pin. This includes electrical design guidelines for the ETS# pin and
usage/placement guidelines of the thermal sensors for maximum effectiveness. Current third party
vendor product offerings that may be suitable for the ETS# pin application include ambient temperature
thermal sensors and remote diode thermal sensors. Also, thermal sensors that implement an open-drain
output for signaling a thermal event would provide the most flexibility from an electrical and for layout
design perspective.
6.9.1.
ETS# Usage Model
The thermal sensors targeted for this application with the GMCH's ETS# are planned to be capable of
measuring the ambient temperature only and should be able to assert ETS# if the preprogrammed
thermal limits/conditions are met or exceeded. Because many variables within a mobile system can
affect the temperature measured at any given point in a system, the expected usage and effectiveness of
ETS# is also very focused. Because of factors such as thermal sensor placement, airflow within a mobile
chassis, adjacent components, thermal sensor sensitivity, and thermal sensor response time, ETS# can
effectively be used for controlling skin temperatures. However, due to the location of the thermal sensor,
ETS# should not be used for measuring or controlling the Tj or Tcase parameters of DDR-SDRAM
devices since it cannot respond quickly enough to dynamic changes in DRAM power.
6.9.2.
ETS# Design Guidelines
ETS#, as implemented in the GMCH, is an active low signal and does not have an integrated pull-up to
maintain a logic 1. As a result of this, an external 8.2 kΩ to 10 kΩ pull-up resistor should be provided
near the ETS# pin, connected to 3.3V. Ideally, the thermal sensor should implement an open drain type
output buffer to drive ETS#. A system is expected to have one thermal sensor per SO-DIMM connector
on the motherboard.
6.9.3.
Thermal Sensor Routing and Placement Guidelines
Routing guidelines and other special, motherboard design considerations will vary with the vendor and
type of thermal sensor chosen for this ETS# application. As a result, vendor specific design guidelines
should also be followed closely to ensure proper operation of this feature. As a general rule, system
designers should follow good design practices in ensuring good signal integrity on this signal as well as
achieving adequate isolation from adjacent signals. Also, any thermal design considerations (e.g. proper
ground flood placement underneath the external thermal sensor; proper isolation of the differential
signal routing for thermal diode applications, etc.) for the external thermal sensor itself should also be
met.
The many factors that can affect the accuracy of ambient temperature measurements by thermal sensors
make the placement of them a very critical and especially challenging task. Ideally, one thermal sensor
should be placed near each SO-DIMM in a system. The thermal sensor should be located in an area
where the effects of airflow and effects of conduction from adjacent components are minimized. This
®
Intel
855GM/855GME Chipset Platform Design Guide
System Memory Design Guidelines (DDR-SDRAM) for SO-DIMM configuration
117
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