Chapter 10. Power and Thermal Management
10-9
Thermal Assist Unit (TAU)
The bit Telds in the THRM3 SPR are described in Table 10-3.
Table 10-3. THRM3 Bit Field Settings
10.3.2 Thermal Assist Unit Operation
The thermal assist unit can be programmed to operate in single- or dual-threshold modes,
which results in the thermal assist unit generating a thermal management interrupt when
one or both threshold values are crossed. In addition, with the appropriate software routine,
the thermal assist unit can also directly determine the junction temperature. The following
sections describe the conTguration of the thermal assist unit to support these modes of
operation.
10.3.2.1 Thermal Assist Unit Single-Threshold Mode
When the thermal assist unit is conTgured for single-threshold mode, either THRM1 or
THRM2 can be used to contain the threshold value, and a thermal management interrupt is
generated when the threshold value is crossed. To conTgure the thermal assist unit for
single threshold operation, set the desired temperature threshold, TID, TIE, and V bits for
either THRM1 or THRM2. The unused THRM
n
threshold SPR should be disabled by
clearing the V bit. In this discussion THRM
n
refers to the THRM threshold SPR (THRM1
or THRM2) selected to contain the active threshold value.
After setting the desired operational parameters, the thermal assist unit is enabled by setting
THRM3[E] and placing a value allowing a sample interval of 20 microseconds or greater
in THRM3[SITV]. The THRM3[SITV] setting determines the number of processor clock
cycles between input to the DAC and sampling of the comparator output; accordingly, the
use of a value smaller than recommended in THRM3[SITV] can cause inaccuracies in the
sensed temperature.
If the junction temperature does not cross the programmed threshold, THRM
n
[TIN] is
cleared to indicate that no interrupt is required and THRM
n
[TIV] is set to indicate that the
TIN bit state is valid. If the threshold value has been crossed, THRM
n
[TIN] and
THRM
n
[TIV] are set and a thermal management interrupt is generated if THRM
n
[TIE] and
MSR[EE] = 1.
A thermal management interrupt is held asserted internally until recognized by the
MPC7400s interrupt unit. Once a thermal management interrupt is recognized, further
Bits
Name
Description
0D17
Reserved for future use. System software should clear these bits.
18D30
SITV
Sample interval timer value. Number of elapsed processor clock cycles before a junction
temperature vs. threshold comparison result is sampled for TIN bit setting and interrupt generation.
This is necessary due to the thermal sensor, DAC, and the analog comparator settling time being
greater than the processor cycle time. The value should be conTgured to allow a 20-μs sampling
interval.
31
E
Enables the thermal sensor compare operation if either THRM1[V] or THRM2[V] = 1.