Chapter 10. Power and Thermal Management
10-5
Programmable Power Modes
10.2.4.2 Nap Mode Bus Snooping Sequence
The MPC7400 also allows dynamic switching between nap and doze mode to allow use of
nap mode without sacriTcing hardware snoop coherency. For this operation, negating
QACK at any time for at least 4 bus cycles guarantees that the MPC7400 has changed from
nap to doze mode in order to snoop. Reasserting QACK then allows the MPC7400 to return
to nap mode. QACK can be reasserted one cycle (or more) after TS is asserted for the snoop.
The MPC7400 re-enters nap mode after it completes all operations necessary to service the
snoop (assuming that QACK was not negated for eight consecutive cycles again). Nap
mode resumes within a few cycles of the snoops address tenure if no snoop push is
required. If a snoop push is required, the MPC7400 does not re-enter nap mode until a few
cycles after the pushs data tenure ends.
It is always necessary to negate QACK for 4 consecutive cycles before presenting a napping
MPC7400 with a snoop, regardless of how many cycles QACK had been asserted
previously. Because of this 4-cycle requirement, a system could try to predict bursts of
snoops by negating QACK for, say, 32 or 64 clocks every time a snoop is presented. In that
way, if a subsequent snoop is within tens of cycles of the previous one, the 4-cycle
requirement would have already been met for that snoop.
The process for handling bus snooping is described as follows:
1. The system negates QACK for four or more bus clock cycles.
2. The MPC7400 snoops address tenures on the bus.
3. The system asserts QACK to restore full nap mode.
10.2.4.3 Returning to Full-Power Mode
The MPC7400 can be returned to full-power mode by asserting INT, SMI, or MCP by
taking a decrementer, performance monitor, or thermal management interrupt, or by
asserting hard reset or soft reset. The transition to full-power takes only a few processor
cycles.
10.2.4.4 Sleep Mode
Sleep mode consumes the least amount of power of the four modes because all functional
units (except the thermal assist and performance monitor units) are disabled. To maximize
power conservation, the PLL may be disabled in the following two ways:
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Placing the PLL_CFG signals in PLL bypass mode and disabling SYSCLK. Note
that forcing SYSCLK into a static state does not disable the MPC7400s PLL, which
continues to operate internally at an undeTned frequency unless placed in PLL
bypass mode. Additionally, if the PLL is enabled, the L2 cache interface DLL
remains locked and the Tve L2 clock signals remain active. The DLL is disabled by
clearing L2CR[L2E].
Placing the PLL_CFG signals in kill mode. In this case, SYSCLK can continue to
operate.
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