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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Power and Thermal Management
Page 346 of 377
gx_10.fm.(1.2)
March 27, 2006
10.4.2.2 TAU Dual-Threshold Mode
The configuration and operation of the TAU’s dual-threshold mode is similar to single-threshold mode, except
both THRM1 and THRM2 are configured with the desired threshold and TID values, and the TIE and V bits
are set to 1. When the THRM3[E] bit is set to 1 to enable temperature measurement and comparison, the first
comparison is made with THRM1. If no thermal-management interrupt results from the comparison, the
number of processor cycles specified in THRM3[SITV] elapses, and the next comparison is made with
THRM2. If no thermal-management interrupt results from the THRM2 comparison, the time specified by
THRM3[SITV] again elapses, and the comparison returns to THRM1.
This sequence of comparisons continues until a thermal-management interrupt occurs, or the TAU is
disabled. When a comparison results in an interrupt, the 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 TAU
ceases making temperature comparisons until an mtspr instruction is executed to one or both of the
threshold SPRs.
10.4.2.3 750GX Junction Temperature Determination
While the 750GX’s TAU 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 find
the junction temperature.
The TAU configuration 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 0 to disable thermal-
management interrupt generation. Once the TAU 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 to 1, 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.
010x1
The junction temperature is less than the threshold, and, as a result, the thermal-
management interrupt is not generated for TIE = 1.
110x1
The junction temperature is greater than the threshold, and, as a result, the thermal-
management interrupt is generated if TIE = 1.
011x1
The junction temperature is greater than the threshold, and, as a result, the thermal-
management interrupt is not generated for TIE = 1.
111x1
The junction temperature is less than the threshold, and, as a result, the thermal-
management interrupt is generated if TIE = 1.
Table 10-3. Valid THRM1 and THRM2 Bit Settings (Page 2 of 2)
TIN
1
TIV
1
TID TIE V Description
Note:
1. The TIN and TIV bits are read-only status bits.