
Chapter 2. Signal Descriptions
2-33
State Meaning
Asserted—Forces the internal processor clocks to continue running,
even if nap mode is active, allowing bus snooping to occur. HALTED
is deasserted to indicate any bus activity and is reasserted to indicate
when the processor is idle and when RUN can be deasserted.
Negated—Internal processor clocks can stop running in nap mode.
Assertion—May occur at any time asynchronously to the input
clocks. The maximum latency between RUN being asserted and the
starting of the internal processor clocks is three bus clock cycles.
Negation—Can occur after the HALTED signal is asserted.
Timing Comments
2.11.7.1 Going from Normal to Doze State (604e)
The only state transition allowed from the normal state is to doze state. This transition
requires system support. The system must assert RUN for at least 10 bus cycles before the
software power management sequence can begin. RUN does not affect 604e operation in
the normal state, but does affect operation during the transition from normal to doze state.
The software power management sequence is the following code:
sync
mtmsr
isync
branch to the
sync
instruction
The
mtmsr
instruction should modify the power management bit MSR[POW] only. All
other MSR values such as the external interrupt enable should be set up before the software
power management sequence is begun. When
mtmsr
is executed, the processor waits for
its internal state to be idle and then asserts HALTED, at which point the processor is in doze
state. When entering doze state, the system must assert RUN for at least 10 bus cycles after
HALTED is asserted. When the processor is in doze state, HALTED is deasserted when a
snoop-triggered write-back is in progress. The system must keep RUN asserted whenever
HALTED is deasserted in doze mode due to a snoop write-back operation.
If the software power management sequence is initiated from the normal state with RUN
not asserted, the processor would attempt to go directly to nap state. This transition is not
supported and may cause the system to hang later when the processor leaves nap state.
2.11.7.2 Going from Doze to Nap State
For the processor to go from doze to nap state, the system must first ensure that the bus is
idle and that HALTED is asserted for at least 10 bus cycles. The system should then deassert
RUN and continue to prevent bus grants for at least 10 additional bus cycles, at which point
the processor is in nap state and bus transactions can be resumed. The processor does not
snoop any subsequent bus transactions.
In going from doze to nap state, the 604e must see the bus idle, which here means that the
604e cannot receive any TS or XATS assertions. The system can ensure this by negating
address bus grants to other bus devices.