Atmel SAM9X25, Manual

Page: 80 / 1145

80
11054A–ATARM–27-Jul-11
SAM9X25
80
11054A–ATARM–27-Jul-11
SAM9X25
progress, it is delayed until the software indicates to the AIC the end of the current service by
writing the AIC_EOICR (End of Interrupt Command Register).
The write of AIC_EOICR is the
exit point of the interrupt handling .
13.8.3.2 Interrupt Nesting
The priority controller utilizes interrupt nesting in order for the high priority interrupt to be handled
during the service of lower priority interrupts. This requires the interrupt service routines of the
lower interrupts to re-enable the interrupt at the processor level.
When an interrupt of a higher priority happens during an already occurring interrupt service rou-
tine, the nIRQ line is re-asserted. If the interrupt is enabled at the core level, the current
execution is interrupted and the new interrupt service routine should read the AIC_IVR. At this
time, the current interrupt number and its priority level are pushed into an embedded hardware
stack, so that they are saved and restored when the higher priority interrupt servicing is finished
and the AIC_EOICR is written.
The AIC is equipped with an 8-level wide hardware stack in order to support up to eight interrupt
nestings pursuant to having eight priority levels.
13.8.3.3 Interrupt Vectoring
The interrupt handler addresses corresponding to each interrupt source can be stored in the reg-
isters AIC_SVR1 to AIC_SVR31 (Source Vector Register 1 to 31). When the processor reads
AIC_IVR (Interrupt Vector Register), the value written into AIC_SVR corresponding to the cur-
rent interrupt is returned.
This feature offers a way to branch in one single instruction to the handler corresponding to the
current interrupt, as AIC_IVR is mapped at the absolute address 0xFFFF F100 and thus acces-
sible from the ARM interrupt vector at address 0x0000 0018 through the following instruction:
LDR PC,[PC,# -&F20]
When the processor executes this instruction, it loads the read value in AIC_IVR in its program
counter, thus branching the execution on the correct interrupt handler.
This feature is often not used when the application is based on an operating system (either real
time or not). Operating systems often have a single entry point for all the interrupts and the first
task performed is to discern the source of the interrupt.
However, it is strongly recommended to port the operating system on AT91 products by support-
ing the interrupt vectoring. This can be performed by defining all the AIC_SVR of the interrupt
source to be handled by the operating system at the address of its interrupt handler. When doing
so, the interrupt vectoring permits a critical interrupt to transfer the execution on a specific very
fast handler and not onto the operating system’s general interrupt handler. This facilitates the
support of hard real-time tasks (input/outputs of voice/audio buffers and software peripheral han-
dling) to be handled efficiently and independently of the application running under an operating
system.
13.8.3.4 Interrupt Handlers
This section gives an overview of the fast interrupt handling sequence when using the AIC. It is
assumed that the programmer understands the architecture of the ARM processor, and espe-
cially the processor interrupt modes and the associated status bits.
It is assumed that: