Thermal Management - IBM PowerPC 750GX User Manual

Nap
Sleep
In addition, the 750GX allows software-controlled toggling between two operating frequencies. During periods
of processor inactivity or for applications requiring reduced computing performance, the processor may be
toggled to a lower frequency to conserve power.
Chapter 10, Power and Thermal Management, on page 335 provides information about power-saving and
thermal-management modes for the 750GX.

1.11 Thermal Management

The 750GX'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 manage-
ment 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 750GX's on-chip thermal sensor and logic tightly couple the thermal-manage-
ment implementation.
The TAU consists of a thermal sensor, digital-to-analog convertor, comparator, control logic, and the dedi-
cated SPRs described in Section 1.4, PowerPC Registers and Programming Model, on page 42. 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 using a software successive approximation rou-
tine.
gx_01.fm.(1.2)
March 27,2006
The nap mode further reduces power consumption by disabling bus snooping,
leaving only the Time Base Register and the PLL in a powered state. The 750GX
returns to the full-power state upon receipt of an external asynchronous interrupt, a
system management interrupt, a decrementer exception, a hard or soft reset, or a
machine-check interrupt (MCP). A return to full-power state from nap state takes
only a few processor clock cycles. When the processor is in nap mode, if QACK is
negated, the processor is put in doze mode to support snooping.
Sleep mode minimizes power consumption by disabling all internal functional units,
after which external system logic can disable the PLL and SYSCLK. Returning the
750GX to the full-power state requires enabling the PLL and SYSCLK, followed by
the assertion of an external asynchronous interrupt, a system management inter-
rupt, a hard or soft reset, or a machine-check interrupt (MCP) signal after the time
required to relock the PLL.
IBM PowerPC 750GX and 750GL RISC Microprocessor
User's Manual
PowerPC 750GX Overview
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