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Chapter 4 System Support
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
4-54
4.8.5 TEMPERATURE SENSING
This system employs two sensors for monitoring the temperature inside the chassis. A thermister
attached to the heat sink of the Pentium II SEC cartridge is used to detect the caution level. This
thermister, connected to the system board through header P15, is part of sensing logic that
provides input to a Compaq ASIC. The sensing logic is set to trip when 179.6 °F (82 °C) is
reached. At that time the Compaq ASIC can generate an SMI (if so configured, see registers
below) resulting in a warning to the user and/or the chassis fan being turned on. Three general
purpose input ports of the 82371south bridge monitor status of the Thermal Caution circuitry.
They are listed below with their default values:
1. 82371 GPI #19 – Chassis fan connection (1 = fan connected)
2. 82371 GPI #20 – Thermal Caution event status (0 = caution event has not occurred)
3. 82371 GPI #21 – Thermal sensor connection status (0 = connected)
The Pentium II processor contains a sensor utilized to detect a deadly condition. When the
processor temperature reaches 135 °F the THERMTRIP- signal is asserted and recorded in a
Compaq ASIC (see following registers). Assertion of THERMTRIP- also results in turning off
the system’s clock generator, effectively shutting down the system.
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 Reserved
NOTE: Bits 1,0 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.
1,0 Reserved