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Technical Reference Guide
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition - September1997
4-47
4.7.5 TEMPERATURE SENSING
Two components (one programmable LM75 and one TC623) are used in monitoring the internal
temperature of the system. The LM75 sensor is mounted in the cavity of the microprocessor
socket to detect microprocessor temperature. The LM75 is programmed for two temperature
levels:
a. Tos - Overtemperature shutdown value (level at which the LM75’s output becomes
active)
b. Thyst - Hysterious value (level at which the LM75’s output is negated)
In the standard configuration the BIOS programs Tos for 60°C and Thyst for 58°C. Detection by
the LM75 sensor results in a warning being issued to the user and/or the power supply fan being
turned on. Note that upgrading to particular microprocessor step with unique operating
temperature characteristics may require that the BIOS be upgraded as well in order to set the
LM75 to the proper detection levels.
The following two indexed registers are used by BIOS and available to software for controlling
the temperature sense function.
I/O Port C51.02h, Temperature Status/Clear Register
Bit Function
7..2 Reserved
1 Temperature Deadly (RO)
0 = Normal
1 = Critical temperature detected
0 Temperature Caution for Processor 1 (RO)
0 = Normal
1 = High temperature detected at P1
NOTE: Bits 2..0 are cleared when read but will be instantly reset if condition remains.
I/O Port C51.03h, Temperature Interrupt/SMI Enable Register
Bit Function
7..3 Reserved
2 Temperature Deadly Shutdown Disable:
0 = Initiate shutdown w/deadly condition.
1 = Do not initiate shutdown.
1 Temperature SMI Enable:
0 = Do not generate SMI- w/caution condition.
1 = Generate SMI- upon caution condition.
0 Temperature IRQ Enable:
0 = Do not generate IRQ w/ caution condition.
1 = Generate IRQ w/caution condition.
A second sensor (TC623) is used to detect a deadly temperature condition. This sensor, which is
non-programmable (fixed), activates a signal that disables the ICS9147 clock generator,
effectively shutting down the system.