Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet
Product codes
AT91SAM9N12-EK
62
SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]
11063K–ATARM–05-Nov-13
This facilitates the support of hard real-time tasks (input/outputs of voice/audio buffers and software peripheral handling)
to be handled efficiently and independently of the application running under an operating system.
to be handled efficiently and independently of the application running under an operating system.
11.8.3.4 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 especially the processor interrupt modes and the
associated status bits.
programmer understands the architecture of the ARM processor, and especially the processor interrupt modes and the
associated status bits.
It is assumed that:
1.
The Advanced Interrupt Controller has been programmed, AIC_SVR registers are loaded with corresponding inter-
rupt service routine addresses and interrupts are enabled.
rupt service routine addresses and interrupts are enabled.
2.
The instruction at the ARM interrupt exception vector address is required to work with the vectoring
LDR PC, [PC, # -&F20]
When nIRQ is asserted, if the bit “I” of CPSR is 0, the sequence is as follows:
1.
The CPSR is stored in SPSR_irq, the current value of the Program Counter is loaded in the Interrupt link register
(R14_irq) and the Program Counter (R15) is loaded with 0x18. In the following cycle during fetch at address 0x1C,
the ARM core adjusts R14_irq, decrementing it by four.
(R14_irq) and the Program Counter (R15) is loaded with 0x18. In the following cycle during fetch at address 0x1C,
the ARM core adjusts R14_irq, decrementing it by four.
2.
The ARM core enters Interrupt mode, if it has not already done so.
3.
When the instruction loaded at address 0x18 is executed, the program counter is loaded with the value read in
AIC_IVR. Reading the AIC_IVR has the following effects:
AIC_IVR. Reading the AIC_IVR has the following effects:
Sets the current interrupt to be the pending and enabled interrupt with the highest priority. The current level
is the priority level of the current interrupt.
is the priority level of the current interrupt.
De-asserts the nIRQ line on the processor. Even if vectoring is not used, AIC_IVR must be read in order to
de-assert nIRQ.
de-assert nIRQ.
Automatically clears the interrupt, if it has been programmed to be edge-triggered.
Pushes the current level and the current interrupt number on to the stack.
Returns the value written in the AIC_SVR corresponding to the current interrupt.
4.
The previous step has the effect of branching to the corresponding interrupt service routine. This should start by
saving the link register (R14_irq) and SPSR_IRQ. The link register must be decremented by four when it is saved
if it is to be restored directly into the program counter at the end of the interrupt. For example, the instruction
saving the link register (R14_irq) and SPSR_IRQ. The link register must be decremented by four when it is saved
if it is to be restored directly into the program counter at the end of the interrupt. For example, the instruction
SUB
PC, LR, #4
may be used.
5.
Further interrupts can then be unmasked by clearing the “I” bit in CPSR, allowing re-assertion of the nIRQ to be
taken into account by the core. This can happen if an interrupt with a higher priority than the current interrupt
occurs.
taken into account by the core. This can happen if an interrupt with a higher priority than the current interrupt
occurs.
6.
The interrupt handler can then proceed as required, saving the registers that will be used and restoring them at the
end. During this phase, an interrupt of higher priority than the current level will restart the sequence from step 1.
end. During this phase, an interrupt of higher priority than the current level will restart the sequence from step 1.
Note:
If the interrupt is programmed to be level sensitive, the source of the interrupt must be cleared during this phase.
7.
The “I” bit in CPSR must be set in order to mask interrupts before exiting to ensure that the interrupt is completed
in an orderly manner.
in an orderly manner.
8.
The End of Interrupt Command Register (AIC_EOICR) must be written in order to indicate to the AIC that the cur-
rent interrupt is finished. This causes the current level to be popped from the stack, restoring the previous current
level if one exists on the stack. If another interrupt is pending, with lower or equal priority than the old current level
but with higher priority than the new current level, the nIRQ line is re-asserted, but the interrupt sequence does not
immediately start because the “I” bit is set in the core. SPSR_irq is restored. Finally, the saved value of the link reg-
ister is restored directly into the PC. This has the effect of returning from the interrupt to whatever was being
executed before, and of loading the CPSR with the stored SPSR, masking or unmasking the interrupts depending
on the state saved in SPSR_irq.
rent interrupt is finished. This causes the current level to be popped from the stack, restoring the previous current
level if one exists on the stack. If another interrupt is pending, with lower or equal priority than the old current level
but with higher priority than the new current level, the nIRQ line is re-asserted, but the interrupt sequence does not
immediately start because the “I” bit is set in the core. SPSR_irq is restored. Finally, the saved value of the link reg-
ister is restored directly into the PC. This has the effect of returning from the interrupt to whatever was being
executed before, and of loading the CPSR with the stored SPSR, masking or unmasking the interrupts depending
on the state saved in SPSR_irq.
Note:
The “I” bit in SPSR is significant. If it is set, it indicates that the ARM core was on the verge of masking an inter-
rupt when the mask instruction was interrupted. Hence, when SPSR is restored, the mask instruction is
completed (interrupt is masked).
rupt when the mask instruction was interrupted. Hence, when SPSR is restored, the mask instruction is
completed (interrupt is masked).