Chapter 10. Power and Thermal Management
10-11
Instruction Cache Throttling
comparison with the threshold SPR causing the interrupt is halted, but comparisons
continue with the other threshold SPR. Following a thermal management interrupt, the
interrupt service routine must read both THRM1 and THRM2 to determine which threshold
was crossed. Note that it is possible for both threshold values to have been crossed, in which
case the thermal assist unit stops making temperature comparisons until an
mtspr
instruction is executed to one or both of the threshold SPRs.
10.3.2.3 MPC7400 Junction Temperature Determination
While the MPC7400s thermal assist unit does not implement an analog-to-digital converter
to enable the direct determination of the junction temperature, system software can execute
a simple successive approximation routine to Tnd the junction temperature.
The thermal assist unit conTguration used to approximate the junction temperature is the
same required for single-threshold mode, except that the threshold SPR selected has its TIE
bit cleared to disable thermal management interrupt generation. Once the thermal assist unit
is enabled, the successive approximation routine loads a threshold value into the active
threshold SPR, and then continuously polls the threshold SPRs TIV bit until it is set,
indicating a valid TIN bit. The successive approximation routine can then evaluate the TIN
bit value, and then increment or decrement the threshold value for another comparison. This
process is continued until the junction temperature is determined.
10.3.2.4 Power Saving Modes and Thermal Assist Unit Operation
The static power saving modes (nap, doze, and sleep) allow the processor temperature to be
lowered quickly and can be invoked through the use of the thermal assist unit and associated
thermal management interrupt. The thermal assist unit remains operational in nap and doze
modes, and it remains operational in sleep mode as long as the SYSCLK input remains
active. If SYSCLK is made static when sleep mode is invoked, the thermal assist unit is
rendered inactive. If the MPC7400 is entering sleep mode with SYSCLK disabled, the
thermal assist unit should be conTgured to disable thermal management interrupts to avoid
an unwanted thermal management interrupt when the SYSCLK input signal is restored.
10.4 Instruction Cache Throttling
The MPC7400 provides an instruction cache throttling mechanism to effectively reduce the
instruction execution rate without the complexity and overhead of dynamic clock control.
When used with the thermal assist unit and dynamic power management, instruction cache
throttling provides the system designer with a exible way to control device temperature
while allowing the processor to continue operating.
The instruction cache throttling mechanism simply reduces the instruction forwarding rate
from the instruction cache to the instruction dispatcher. Normally, the instruction cache
forwards four instructions to the instruction dispatcher every clock cycle if all the
instructions hit in the cache. For thermal management, the MPC7400 provides a
supervisor-level instruction cache throttling control SPR (ICTC). The instruction