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Chapter 18 Interrupt Controller (INTC)
MPC5606BK Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor
337
For example, ISR1 executes every 100 µs, ISR2 executes every 200 µs, and ISR3 executes every 300 µs.
ISR1 has a higher priority than ISR, 2 which has a higher priority than ISR3; however, if ISR3 has a
deadline of 150 µs, then it has a higher priority than ISR2.
The INTC has 16 priorities, which may be less than the number of ISRs. In this case, the ISRs should be
grouped with other ISRs that have similar deadlines. For example, a priority could be allocated for every
time the request rate doubles. ISRs with request rates around 1 ms would share a priority, ISRs with request
rates around 500 µs would share a priority, ISRs with request rates around 250 µs would share a priority,
etc. With this approach, a range of ISR request rates of 2
16
could be included, regardless of the number of
ISRs.
Reducing the number of priorities reduces the processor’s ability to meet its deadlines. However, reducing
the number of priorities can reduce the size and latency through the interrupt controller. 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.
18.7.7
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 they may also be used by processors to interrupt other processors in
a multiple processor system.
18.7.7.1
Scheduling a lower priority portion of an ISR
A portion of an ISR needs to be executed at the PRI
x
value in the INTC Priority Select Registers
(INTC_PSR0_3–INTC_PSR232_233), which becomes the PRI value in INTC_CPR with the interrupt
acknowledge. The ISR, however, can have a portion that does not need to be executed at this higher
priority. Therefore, executing the later portion that does not need to be executed at this higher priority can
prevent the execution of ISRs that 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 is for the ISR, after completing the higher
priority portion, to set a SET
x
bit in INTC_SSCIR0_3–INTC_SSCIR4_7. Writing a 1 to SET
x
causes a
software configurable interrupt request. This software configurable interrupt request will usually have a
lower PRI
x
value in the INTC_PSR
x
_
x
and will not cause preemptive scheduling inefficiencies. After
generating a software settable interrupt request, the higher priority ISR completes. The lower priority ISR
is scheduled according to its priority. Execution of the higher priority ISR is not resumed after the
completion of the lower priority ISR.
18.7.7.2
Scheduling an ISR on another processor
Because the SET
x
bits in the INTC_SSCIR
x
_
x
are memory mapped, processors in multiple-processor
systems can schedule ISRs on the other processors. One application is that one processor wants to
command another processor to perform a piece of work and the initiating processor does not need to use
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