AT91SAM7S32-AI ATMEL [ATMEL Corporation], AT91SAM7S32-AI Datasheet - Page 138

• Manufacturer Part Number: AT91SAM7S32-AI
• Description: AT91 ARM Thumb-based Microcontrollers
• Manufacturer: ATMEL [ATMEL Corporation]

Protect Mode
138
AT91SAM7S32 Preliminary
Figure 52. Fast Forcing
The Protect Mode permits reading the Interrupt Vector Register without performing the associ-
ated 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 applica-
tions and updates the opened windows, it might read the AIC User Interface and thus the IVR.
This has undesirable consequences:
•
•
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 undes-
ired state.
This is avoided by using the Protect Mode. Writing DBGM in AIC_DCR (Debug Control Regis-
ter) 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 rou-
tine 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 opera-
tions 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.
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.
Source 0 _ FIQ
Source n
Automatic Clear
Automatic Clear
Input Stage
Input Stage
Read IVR if Source n is the current interrupt
and if Fast Forcing is disabled on Source n.
Read FVR if Fast Forcing is
disabled on Sources 1 to 31.
AIC_IPR
AIC_IPR
AIC_IMR
AIC_IMR
AIC_FFSR
Manager
Priority
6071A–ATARM–28-Oct-04
nIRQ
nFIQ