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82854 Graphics Memory Controller Hub (GMCH)
150 D15343-003
7.4 External Thermal Sensor Input
An External Thermal sensor with a serial interface may be placed next to DDR SDRAM DIMM (or
any other appropriate platform location), or a remote Thermal Diode may be placed next to the
DDR DIMM (or any other appropriate platform location) and connected to the External Thermal
sensor. Intel advises that the External Thermal sensor contains some form of hysteresis, since none
is provided by the GMCH hardware.
The external sensor can be connected to the ICH4-M via the SMBus interface to allow
programming and setup by BIOS software over the serial interface. The External sensor's output
should include an Active-Low Open-Drain signal indicating an Over-Temp condition, which
remains asserted for as long as the Over-Temp Condition exists, and deasserts when temperature
has returned to within normal operating range. This External sensor output will be connected to the
GMCH input (EXTTS_0) and will trigger a Preset Interrupt and/or Read-Throttle on a level-
sensitive basis.
Additional External Thermal sensor's outputs, for multiple sensors, can be wire-OR'ed together
allow signaling from multiple sensors located physically separately. Software can, if necessary,
distinguish which DDR DIMM(s) is the source of the over-temp through the serial interface.
However, since the DDR DIMM(s) will be located on the same System Memory Bus Data lines,
any GMCH-based Read Throttle will apply equally.
Note: The use of external sensors that include an internal pull-up resistor on the open-drain Thermal trip
output is discouraged. However, it may be possible depending on the size of the pull-up and the
voltage of the sensor. Please refer to the Intel® 854 Chipset Platform Design Guide For Use with
Ultra Low Voltage Intel® Celeron® M Processor at 600 MHz (contact your Intel representative for
the latest version of this document).
7.4.1 Usage
External sensor(s) used for dynamic temperature feedback control:
• Sensor on DDR DIMMs, which can be used to dynamically control read throttling.