System Memory Design Guidelines (DDR-SDRAM)
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Intel
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855PM Chipset Platform Design Guide
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6.8.
External Thermal Sensor Based Throttling (ETS#)
The Intel 855PM MCH’s ETS# input pin is an active low input that can be used with an external thermal
sensor to monitor the temperature of the DDR SO-DIMMs for a possible thermal condition. Assertion of
ETS# will result in the limiting of DRAM bandwidth on the DDR memory interface to reduce the
temperature in the vicinity of the system memory.
By default, the functionality and input buffer associated with ETS# are disabled. Also, the MCH 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 MCH 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 layout design perspective.
6.8.1.
ETS# Usage Model
The thermal sensors targeted for this application with the Intel 855PM MCH’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.
