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MPC7400 RISC Microprocessor Users Manual
Thermal Management
Chapter 10, òPower and Thermal Management,ó provides information about power saving
and thermal management modes for the MPC7400.
1.11 Thermal Management
The MPC7400s thermal assist unit (TAU) provides a way to control heat dissipation. This
ability is particularly useful in portable computers, which, due to power consumption and
size limitations, cannot use desktop cooling solutions such as fans. Therefore, better heat
sink designs coupled with intelligent thermal management is of critical importance for high
performance portable systems.
Primarily, the thermal management system monitors and regulates the systems operating
temperature. For example, if the temperature is about to exceed a set limit, the system can
be made to slow down or even suspend operations temporarily in order to lower the
temperature.
The thermal management facility also ensures that the processors junction temperature
does not exceed the operating speciTcation. To avoid the inaccuracies that arise from
measuring junction temperature with an external thermal sensor, the MPC7400s on-chip
thermal sensor and logic tightly couples the thermal management implementation.
The TAU consists of a thermal sensor, digital-to-analog convertor, comparator, control
logic, and the dedicated SPRs described in Section 1.4, òPowerPC Registers and
Programming Model.ó The TAU does the following:
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Compares the junction temperature against user-programmable thresholds
Generates a thermal management interrupt if the temperature crosses the threshold
Enables the user to estimate the junction temperature by way of a software
successive approximation routine
The TAU is controlled through the privileged
mtspr
/
mfspr
instructions to the three SPRs
provided for conTguring and controlling the sensor control logic, which function as
follows:
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THRM1 and THRM2 provide the ability to compare the junction temperature
against two user-provided thresholds. Having dual thresholds gives the thermal
management software Tner control of the junction temperature. In single threshold
mode, the thermal sensor output is compared to only one threshold in either THRM1
or THRM2.
THRM3 is used to enable the TAU and to control the comparator output sample
time. The thermal management logic manages the thermal management interrupt
generation and time multiplexed comparisons in the dual threshold mode as well as
other control functions.
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