Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

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SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

The Protect Mode permits reading the Interrupt Vector Register without performing the associated automatic operations.
This is necessary when working with a debug system. When a debugger, working either with a Debug Monitor or the
ARM processor's ICE, stops the applications and updates the opened windows, it might read the AIC User Interface and
thus the IVR. This has undesirable consequences:



If an enabled interrupt with a higher priority than the current one is pending, it is stacked. 



If there is no enabled pending interrupt, the spurious vector is returned.

In either case, an End of Interrupt command is necessary to acknowledge and to restore the context of the AIC. This
operation is generally not performed by the debug system as the debug system would become strongly intrusive and
cause the application to enter an undesired state.

This is avoided by using the Protect Mode. Writing PROT in AIC_DCR (Debug Control Register) at 0x1 enables the
Protect Mode. 

When the Protect Mode is enabled, the AIC performs interrupt stacking only when a write access is performed on the
AIC_IVR. Therefore, the Interrupt Service Routines must write (arbitrary data) to the AIC_IVR just after reading it. The
new context of the AIC, including the value of the Interrupt Status Register (AIC_ISR), is updated with the current
interrupt only when AIC_IVR is written. 

An AIC_IVR read on its own (e.g., by a debugger), modifies neither the AIC context nor the AIC_ISR. Extra AIC_IVR
reads perform the same operations. However, it is recommended to not stop the processor between the read and the
write of AIC_IVR of the interrupt service routine to make sure the debugger does not modify the AIC context.

To summarize, in normal operating mode, the read of AIC_IVR performs the following operations within the AIC:

1.

Calculates active interrupt (higher than current or spurious).

2.

Determines and returns the vector of the active interrupt.

3.

Memorizes the interrupt.

4.

Pushes the current priority level onto the internal stack.

5.

Acknowledges the interrupt.

However, while the Protect Mode is activated, only operations 1 to 3 are performed when AIC_IVR is read. Operations 4
and 5 are only performed by the AIC when AIC_IVR is written. 

Software that has been written and debugged using the Protect Mode runs correctly in Normal Mode without
modification. However, in Normal Mode the AIC_IVR write has no effect and can be removed to optimize the code.

The Advanced Interrupt Controller features protection against spurious interrupts. A spurious interrupt is defined as being
the assertion of an interrupt source long enough for the AIC to assert the nIRQ, but no longer present when AIC_IVR is
read. This is most prone to occur when:



An external interrupt source is programmed in level-sensitive mode and an active level occurs for only a short time.



An internal interrupt source is programmed in level sensitive and the output signal of the corresponding embedded 
peripheral is activated for a short time. (As in the case for the Watchdog.)



An interrupt occurs just a few cycles before the software begins to mask it, thus resulting in a pulse on the interrupt 
source.

The AIC detects a spurious interrupt at the time the AIC_IVR is read while no enabled interrupt source is pending. When
this happens, the AIC returns the value stored by the programmer in AIC_SPU (Spurious Vector Register). The
programmer must store the address of a spurious interrupt handler in AIC_SPU as part of the application, to enable an as
fast as possible return to the normal execution flow. This handler writes in AIC_EOICR and performs a return from
interrupt.