10-8
MPC7400 RISC Microprocessor Users Manual
Thermal Assist Unit (TAU)
The thermal assist unit provides thermal control by periodically comparing the MPC7400s
junction temperature against user-programmed thresholds, and generating a thermal
management interrupt if the threshold values are reached. The thermal assist unit also
enables the user to determine the junction temperature through a software successive
approximation routine.
The thermal assist unit is controlled through three supervisor-level SPRs, accessed through
the
mtspr
/
mfspr
instructions. Two SPRs (THRM1 and THRM2) provide temperature
threshold values that can be compared to the junction temperature value, and control bits
that enable comparison and thermal interrupt generation. The third SPR (THRM3) provides
a thermal assist unit enable bit and a sample interval timer. Note that all the bits in THRM1,
THRM2, and THRM3 are cleared during a hard reset, and the thermal assist unit remains
idle and in a low-power state until conTgured and enabled.
The Telds in the THRM1 and THRM2 SPRs are described in Table 10-2.
Table 10-2. THRM1 and THRM2 Field Descriptions
Bits
Field
Description
0
TIN
Thermal management interrupt bit. Read only. Set if the thermal sensor output crosses the
threshold speciTed in the SPR. The state of this bit is valid only if TIV is set. The interpretation of
the TIN bit is controlled by the TID bit.
1
TIV
Thermal management interrupt valid. Read only. This bit is set by the thermal assist logic to
indicate that the thermal management interrupt (TIN) state is valid.
2D8
Threshold
Threshold value that the output of the thermal sensor is compared to. The threshold range is
between 0 and 127C, and each bit represents 1C. Note that this is not the resolution of the
thermal sensor.
9D28
Reserved, for future use. The MPC7400 always returns zeros on a read of these bits. For
maximum exibility with other implementations, system software should clear these bits when
writing to the ICTC.
29
TID
Thermal management interrupt direction bit. Selects the result of the temperature comparison to
set TIN. If TID = 0, TIN is set and an interrupt occurs if the junction temperature
exceeds the
threshold. If TID = 1, TIN is set and an interrupt is indicated if the junction temperature is below the
threshold.
30
TIE
Thermal management interrupt enable. Enables assertion of the thermal management interrupt
signal. The thermal management interrupt is maskable by the MSR[EE] bit. If TIE = 0 and THRM
n
is valid, the TIN bit records the status of the junction temperature vs. threshold comparison
without asserting an interrupt signal. This feature allows system software to make a successive
approximation to estimate the junction temperature.
31
V
SPR valid bit. Set to indicate that the SPR contains valid threshold, TID, and TIE control bits.
Setting THRM1[V], THRM2[V], and THRM3[E] enables operation of the thermal sensor.