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Interrupt Controller (INTC)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
377
15.6.6
Selecting priorities according to request rates and
deadlines
The selection of the priorities for the ISRs can be made using Rate Monotonic Scheduling (RMS) or a
superset of it, Deadline Monotonic Scheduling (DMS). In RMS, the ISRs that have higher request rates
have higher priorities. In DMS, if the deadline is before the next time the ISR is requested, then the ISR is
assigned a priority according to the time from the request for the ISR to the deadline, not from the time of
the request for the ISR to the next request for it.
For example, ISR1 executes every 100
s, ISR2 executes every 200
s, and ISR3 executes every 300
s.
ISR1 has a higher priority than ISR2, which has a higher priority than ISR3. However, if ISR3 has a
deadline of 150
s, then it has a higher priority than ISR2.
The INTC has 16 priorities, which can be considerably less than the number of ISRs. In this case, group
the ISRs with other ISRs that have similar deadlines. For example, when a priority is allocated for every
time, the request rate doubles ISRs with request rates around 1 ms share a priority; ISRs with request rates
around 500
s share a priority; ISRs with request rates around 250
s share a priority, etc. With this
approach, a range of ISR request rates of 2
16
can be covered, regardless of the number of ISRs.
Reducing the number of priorities reduces the processor's ability to meet its deadlines. However, it also
allows easier management of ISRs with similar deadlines that share a resource. They do not need to use
the PCP to access the shared resource.
15.6.7
Software configurable interrupt requests
The software configurable interrupt requests can be used in two ways. They can be used to schedule a
lower priority portion of an ISR and for processors to interrupt other processors in a multiple processor
system.
15.6.7.1
Scheduling a lower priority portion of an ISR
A portion of an ISR needs to be executed at the PRI
n
value in INTC priority select registers
(INTC_PSR0–INTC_PSR485), which becomes the PRI value in INTC current priority register
(INTC_CPR) with the interrupt acknowledgement. The ISR, however, can have a portion of it that does
not need to be executed at this higher priority. Therefore, executing this later portion that does not need to
be executed at this higher priority can prevent the execution of ISRs, which do not have a higher priority
than the earlier portion of the ISR but do have a higher priority than what the later portion of the ISR needs.
This preemptive scheduling inefficiency reduces the processor's ability to meet its deadlines.
One option is for the ISR to complete the earlier higher priority portion, but then schedule through the
RTOS a task to execute the later lower priority portion. However, some RTOSs can require a large amount
of time for an ISR to schedule a task. Therefore, a second option for the ISR is, after completing the higher
priority portion, to set a SET
n
bit in INTC software set/clear interrupt registers
(INTC_SSCIR0–INTC_SSCIR7). Writing a 1 to SET
n
causes a software configurable interrupt request.
This software configurable interrupt request, which usually has a lower PRI
n
value in the INTC_PSR
n
,
does not cause preemptive scheduling inefficiencies.
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