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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
10-30
Freescale Semiconductor
10.4.2.1.1
Priority Arbitrator Submodule
The priority arbitrator submodule compares all the priorities of all of the asserted interrupt requests, both
peripheral and software settable. The output of the priority arbitrator submodule is the highest of those
priorities. Also, any interrupt requests which have this highest priority are output as asserted interrupt
requests to the request selector submodule.
10.4.2.1.2
Request Selector Submodule
If only one interrupt request from the priority arbitrator submodule is asserted, then it is passed as asserted
to the vector encoder submodule. If multiple interrupt requests from the priority arbitrator submodule are
asserted, then only the one with the lowest vector is passed as asserted to the vector encoder submodule.
The lower vector is chosen regardless of the time order of the assertions of the peripheral or software
settable interrupt requests.
10.4.2.1.3
Vector Encoder Submodule
The vector encoder submodule generates the unique 9-bit vector for the asserted interrupt request from the
request selector submodule.
10.4.2.1.4
Priority Comparator Submodule
The priority comparator submodule compares the highest priority output from the priority arbitrator
submodule with PRI in INTC_CPR. If the priority comparator submodule detects that this highest priority
is higher than the current priority, then it asserts the interrupt request to the processor. This interrupt request
to the processor asserts whether this highest priority is raised above the value of PRI in INTC_CPR or the
PRI value in INTC_CPR is lowered below this highest priority. This highest priority then becomes the new
priority which will be written to PRI in INTC_CPR when the interrupt request to the processor is
acknowledged. Interrupt requests whose PRI
n
in INTC_PSR
n
are zero will not cause a preemption
because their PRI
n
will not be higher than PRI in INTC_CPR.
One consequence of the priority comparator design is that once a higher priority interrupt is captured, it
must be acknowledged by the CPU before a subsequent interrupt request of even higher priority can be
captured. For example, if the CPU is executing a priority level 1 interrupt, and a priority level 2 interrupt
request is captured by the INTC, followed shortly by a priority level 3 interrupt request to the INTC, the
level 2 interrupt must be acknowledged by the CPU before a new level 3 interrupt will be generated.
10.4.2.2
LIFO
The LIFO stores the preempted PRI values from the INTC_CPR. Therefore, because these priorities are
stacked within the INTC, if interrupts need to be enabled during the ISR, at the beginning of the interrupt
exception handler the PRI value in the INTC_CPR does not need to be loaded from the INTC_CPR and
stored onto the context stack. Likewise at the end of the interrupt exception handler, the priority does not
need to be loaded from the context stack and stored into the INTC_CPR.
The PRI value in the INTC_CPR is pushed onto the LIFO when the INTC_IACKR is read in software
vector mode or the interrupt acknowledge signal from the processor is asserted in hardware vector mode.
The priority is popped into PRI in the INTC_CPR whenever the INTC_EOIR is written.
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