User’s Manual

IBM PowerPC 750GX and 750GL RISC Microprocessor

10. Power and Thermal Management

The 750GX microprocessor is specifically designed for low-power operation. It provides both automatic and program-controlled power reduction modes for progressive reduction of power consumption. It also provides a thermal assist unit (TAU) to allow on-chip thermal measurement, allowing sophisticated thermal management for high-performance portable systems. This chapter describes the hardware support provided by the 750GX for power and thermal management.

10.1 Dynamic Power Management

Dynamic power management (DPM) automatically powers up and down the individual execution units of the 750GX, based upon the contents of the instruction stream. For example, if no floating-point instructions are being executed, the floating-point unit is automatically powered down. Power is not actually removed from the execution unit; instead, each execution unit has an independent clock input, which is automatically controlled on a clock-by-clock basis. Since complementary metal-oxide semiconductor (CMOS) circuits consume negligible power when they are not switching, stopping the clock to an execution unit effectively eliminates its power consumption. The operation of DPM is completely transparent to software or any external hardware. Dynamic power management is enabled by setting the DPM bit in Hardware-Implementation-Dependent Register 0 (HID0[DPM] = 1).

10.2 Programmable Power Modes

The 750GX provides four programmable power modes—full on, doze, nap, and sleep. Software selects these modes by setting one (and only one) of the three power saving mode bits in the HID0 Register.

Hardware can enable a power management state through external asynchronous interrupts. Such a hardware interrupt causes the transfer of program flow to interrupt handler code, which then invokes the appropriate power saving mode. The 750GX also contains a decrementer, which allows it to enter the nap or doze mode for a predetermined amount of time and then return to full power operation through a decrementer interrupt.

Note: The 750GX cannot switch from one power management mode to another without first returning to full- on mode.

The sleep mode disables bus snooping. Therefore, a hardware handshake is provided to ensure coherency before the 750GX enters this power management mode.

These power states and power saving modes are shown Figure 10-1, 750GX Power States and Table 10-1on page 336 summarizes the four power modes.

gx_10.fm.(1.2)

Power and Thermal Management

March 27, 2006

Page 335 of 377