IBM 750GL, 750GX 1.11 Thermal Management

	User’s Manual
	IBM PowerPC 750GX and 750GL RISC Microprocessor
Nap	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	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.

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 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 750GX’s on-chip thermal sensor and logic tightly couple 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, 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)	PowerPC 750GX Overview
March 27,2006	Page 55 of 377