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Chapter 4 Power Management 11
25175H—March 2003 AMD Athlon™ XP Processor Model 8 Data Sheet
Preliminary Information
places the processor into the higher power probe state. After
the Northbridge has completed all probes of the processor, the
Northbridge must disconnect the AMD Athlon system bus
again so that the processor can return to the low-power state.
During the Stop Grant states, the processor latches INIT#,
INTR, NMI, SMI#, or a local APIC interrupt message, if they are
asserted.
The Stop Grant state is exited upon the deassertion of
STPCLK# or the assertion of RESET#. When STPCLK# is
deasserted, the processor initiates a connect of the
AMD Athlon system bus if it is disconnected. After the
processor enters the Working state, any pending interrupts are
recognized and serviced and the processor resumes execution
at the instruction boundary where STPCLK# was initially
recognized. If RESET# is sampled asserted during the Stop
Grant state, the processor exits the Stop Grant state and the
reset process begins.
There are two mechanisms for asserting STPCLK#—hardware
and software.
The Southbridge can force STPCLK# assertion for throttling to
protect the processor from exceeding its maximum case
temperature. This is accomplished by asserting the THERM#
input to the Southbridge. Throttling asserts STPCLK# for a
percentage of a predefined throttling period: STPCLK# is
repetitively asserted and deasserted until THERM# is
deasserted.
Software can force the processor into the Stop Grant state by
accessing ACPI-defined registers typically located in the
Southbridge.
The operating system places the processor into the C2 Stop
Grant state by reading the P_LVL2 register in the Southbridge.
If an ACPI Thermal Zone is defined for the processor, the
operating system can initiate throttling with STPCLK# using
the ACPI defined P_CNT register in the Southbridge. The
Northbridge connects the AMD Athlon system bus, and the
processor enters the Probe state to service cache snoops during
Stop Grant for C2 or throttling.
In C2, probes are allowed, as shown in Figure 3 on page 9