
10-2
G2 PowerPC Core Reference Manual
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Programmable Power Modes
10.3 Programmable Power Modes
Hardware can enable a power management state through external asynchronous interrupts.
The hardware interrupt causes the transfer of program flow to interrupt handler code. The
appropriate mode is then set by the software. The G2 core provides a separate interrupt and
interrupt vector for power management—the system management interrupt (core_smi).
The G2 core also contains a decrementer timer that allows it to enter the nap or doze mode
for a predetermined period and then return to full power operation through the decrementer
interrupt exception.
The G2 core provides four power modes selectable by setting the appropriate control bits
in the MSR and HID0. The four power modes are described briefly as follows:
Full-power—This is the default power state of the G2 core. The G2 core is fully
powered and the internal functional units are operating at the full processor clock
speed. If the dynamic power management mode is enabled, functional units that are
idle will automatically enter a low-power state without affecting performance,
software execution, or external hardware.
Doze—All the functional units of the G2 core are disabled except for the time
base/decrementer registers and the bus snooping logic. When the processor is in
doze mode, an external asynchronous interrupt, system management interrupt,
decrementer exception, hard or soft reset, or machine check input (core_mcp) brings
the G2 core into the full-power state. The core in doze mode maintains the a
phase-locked loop (PLL) in a fully powered state and locked to the system external
clock input (core_sysclk) so a transition to the full-power state takes only a few
processor clock cycles.
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 core returns
to the full-power state upon receipt of an external asynchronous interrupt, system
management interrupt, decrementer exception, hard or soft reset, or machine check
input (core_mcp) signal. A return to full-power state from a nap state takes only a
few processor clock cycles.
Sleep—Sleep mode reduces power consumption to a minimum by disabling all
internal functional units; then external system logic may disable the PLL and
core_sysclk. Returning the core to the full-power state requires the enabling of the
PLL and core_sysclk, followed by the assertion of an external asynchronous
interrupt, system management interrupt, hard or soft reset, or core_mcp signal after
the time required to relock the PLL.
Note that the G2 core cannot switch from one power management mode to another without
first returning to full-on mode. The nap and sleep modes disable bus snooping; therefore, a
hardware handshake using core_qreq and core_qack is provided to ensure coherency before
the core enters these power management modes. Table 10-1 summarizes the four power
states for the core.
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