AT91 datasheet(60/153 Pages) ATMEL

60 / 153 page

AT91X40 Series

1354D–ATARM–08/02

Protect Mode

The Protect Mode permits reading of the Interrupt Vector Register without performing

the associated automatic operations. This is necessary when working with a debug

system.

When a Debug Monitor or an ICE reads the AIC User Interface, the IVR could be read.

This would have the following consequences in Normal Mode.

•

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

be stacked

•

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

In either case, an End of Interrupt command would be necessary to acknowledge and to

restore the context of the AIC. This operation is generally not performed by the debug

system. Hence the debug system would become strongly intrusive, and could cause the

application to enter an undesired state.

This is avoided by using Protect Mode.

The Protect Mode is enabled by setting the AIC bit in the SF Protect Mode Register (see

“SF: Special Function Registers” on page 94).

When 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 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 performed in between the read and the write can cause unpredict-

able results. Therefore, it is strongly recommended not to set a breakpoint between

these two actions, nor to stop the software.

The debug system must not write to the AIC_IVR as this would cause undesirable

effects.

The following table shows the main steps of an interrupt and the order in which they are

performed according to the mode:

Notes:

1. NIRQ de-assertion and automatic interrupt clearing if the source is programmed as

level sensitive.

2. Software that has been written and debugged using Protect Mode will run 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.

Action

Normal Mode

Protect Mode

Calculate active interrupt (higher than current or spurious)

Read AIC_IVR

Determine and return the vector of the active interrupt

Read AIC_IVR

Memorize interrupt

Read AIC_IVR

Push on internal stack the current priority level

Read AIC_IVR

Write AIC_IVR

Acknowledge the interrupt (1)

Read AIC_IVR

Write AIC_IVR

No effect(2)

Write AIC_IVR

–

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