Interrupts and Interrupt Controller (INTC)
PXN20 Microcontroller Reference Manual, Rev. 1
10-40 Freescale Semiconductor
code to create stack frame, save working register, and save SRR0 and SRR1
wrteei 1 # enable processor recognition of interrupts
code to save rest of context required by e500 EABI
bl ISRx # branch to ISR for interrupt with vector x
epilog:
code to restore most of context required by e500 EABI
# Popping the LIFO after the restoration of most of the context and the disabling of processor
# recognition of interrupts eases the calculation of the maximum stack depth at the cost of
# postponing the servicing of the next interrupt request.
mbar # ensure store to clear flag bit has completed
lis r3,INTC_EOIR_PRCn@ha # form adjusted upper half of INTC_EOIR_PRCn address
li r4,0x0 # form 0 to write to INTC_EOIR_PRCn
wrteei 0 # disable processor recognition of interrupts
stw r4,INTC_EOIR_PRCn@l(r3) # store to INTC_EOIR_PRCn, informing INTC to lower priority
code to restore SRR0 and SRR1, restore working registers, and delete stack frame
rfi
ISRx:
code to service the interrupt event
code to clear flag bit which drives interrupt request to INTC
blr # branch to epilog
10.5.3 ISR, RTOS, and Task Hierarchy
The RTOS and all of the tasks under its control typically execute with PRI in INTC current priority register
(INTC_CPR_PRC0 or INTC_CPR_PRC1) having a value of 0. The RTOS executes the tasks according
to whatever priority scheme it may have, but that priority scheme is independent and has a lower priority
of execution than the priority scheme of the INTC. In other words, the ISRs execute above
INTC_CPR_PRCn priority 0 and outside the control of the RTOS, the RTOS executes at
INTC_CPR_PRCn priority 0, and while the tasks execute at different priorities under the control of the
RTOS, they also execute at INTC_CPR_PRCn priority 0.
If a task shares a resource with an ISR and the PCP is being used to manage that shared resource, then the
task’s priority can be elevated in the INTC_CPR_PRCn while the shared resource is being accessed.
An ISR whose PRIn in INTC priority select registers (INTC_PSR0–INTC_PSR315) has a value of 0 does
not cause an interrupt request to the selected processor, even if its peripheral or software settable interrupt
request is asserted. For a peripheral interrupt request, not setting its enable bit or disabling the mask bit
causes it to remain negated, which consequently also does not cause an interrupt request to the processor.
Since the ISRs are outside the control of the RTOS, this ISR does not run unless called by another ISR or
the interrupt exception handler, perhaps after executing another ISR.
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