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1.11 Thermal Management
The MPC750's 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 system's 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 processor's junction temperature
does not exceed the operating specification. To avoid the inaccuracies that arise from
measuring junction temperature with an external thermal sensor, the MPC750's 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, "PowerPC Registers and
Programming Model." The TAU does the following:
•
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 configuring and controlling the sensor control logic, which function as
follows:
• THRMI and THRM2 provide the ability to compare the junction temperature
against two user-provided thresholds. Having dual thresholds gives the thermal
management software finer control of the junction temperature. In single threshold
mode, the thermal sensor output is compared to only one threshold in either THRMI
orTHRM2.
• 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.
Instruction cache throttling provides control of the MPC750's overall junction temperature
by determining the interval at which instructions are fetched. This feature is accessed
through the ICTC register.
Chapter 10, "Power and Thermal Management," provides information about power saving
and thermal management modes for the MPC750.
Chapter 1. Overview
1-37
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