Freescale Semiconductor MPC5553 - ISR, RTOS, and Task Hierarchy

MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 10-35
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@ha # form adjusted upper half of INTC_EOIR address
li r4,0x0 # form 0 to write to INTC_EOIR
wrteei 0 # disable processor recognition of interrupts
stw r4,INTC_EOIR@l(r3) # store to INTC_EOIR, 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) having a value of 0. The RTOS will execute the tasks according to whatever priority scheme
that 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 priority 0 and outside
the control of the RTOS, the RTOS executes at INTC_CPR priority 0, and while the tasks execute at
different priorities under the control of the RTOS, they also execute at INTC_CPR 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 while the shared resource is being accessed.
An ISR whose PRIn in INTC priority select registers (INTC_PSR0–INTC_PSR307) has a value of 0 will
not cause an interrupt request to the 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 will cause
it to remain negated, which consequently also will not cause an interrupt request to the processor. Because
the ISRs are outside the control of the RTOS, this ISR will not run unless called by another ISR or the
interrupt exception handler, perhaps after executing another ISR.