at91sam9g20-cu ATMEL Corporation, at91sam9g20-cu Datasheet - Page 324

• Manufacturer Part Number: at91sam9g20-cu
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

27.7.5
324
AT91SAM9G20 Preliminary
Protect Mode
All enabled and pending interrupt sources that have the fast forcing feature enabled and that are
programmed in edge-triggered mode must be cleared by writing to the Interrupt Clear Command
Register. In doing so, they are cleared independently and thus lost interrupts are prevented.
The read of AIC_IVR does not clear the source that has the fast forcing feature enabled.
The source 0, reserved to the fast interrupt, continues operating normally and becomes one of
the Fast Interrupt sources.
Figure 27-10. 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 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.
• 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
6384B–ATARM–15-Dec-08
nIRQ
nFIQ