at91sam9263 ATMEL Corporation, at91sam9263 Datasheet - Page 389

• Manufacturer Part Number: at91sam9263
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

28.7.4.5
6249D–ATARM–20-Dec-07
Fast Forcing
Note:
Another way to handle the fast interrupt is to map the interrupt service routine at the address of
the ARM vector 0x1C. This method does not use the vectoring, so that reading AIC_FVR must
be performed at the very beginning of the handler operation. However, this method saves the
execution of a branch instruction.
The Fast Forcing feature of the advanced interrupt controller provides redirection of any normal
Interrupt source on the fast interrupt controller.
Fast Forcing is enabled or disabled by writing to the Fast Forcing Enable Register (AIC_FFER)
and the Fast Forcing Disable Register (AIC_FFDR). Writing to these registers results in an
update of the Fast Forcing Status Register (AIC_FFSR) that controls the feature for each inter-
nal or external interrupt source.
When Fast Forcing is disabled, the interrupt sources are handled as described in the previous
pages.
When Fast Forcing is enabled, the edge/level programming and, in certain cases, edge detec-
tion of the interrupt source is still active but the source cannot trigger a normal interrupt to the
processor and is not seen by the priority handler.
If the interrupt source is programmed in level-sensitive mode and an active level is sampled,
Fast Forcing results in the assertion of the nFIQ line to the core.
If the interrupt source is programmed in edge-triggered mode and an active edge is detected,
Fast Forcing results in the assertion of the nFIQ line to the core.
The Fast Forcing feature does not affect the Source 0 pending bit in the Interrupt Pending Reg-
ister (AIC_IPR).
2. The ARM core enters FIQ mode.
3. When the instruction loaded at address 0x1C is executed, the program counter is
4. The previous step enables branching to the corresponding interrupt service routine. It is
5. The Interrupt Handler can then proceed as required. It is not necessary to save regis-
6. Finally, the Link Register R14_fiq is restored into the PC after decrementing it by four
the following cycle, during fetch at address 0x20, the ARM core adjusts R14_fiq, decre-
menting it by four.
loaded with the value read in AIC_FVR. Reading the AIC_FVR has effect of automati-
cally clearing the fast interrupt, if it has been programmed to be edge triggered. In this
case only, it de-asserts the nFIQ line on the processor.
not necessary to save the link register R14_fiq and SPSR_fiq if nested fast interrupts
are not needed.
ters R8 to R13 because FIQ mode has its own dedicated registers and the user R8 to
R13 are banked. The other registers, R0 to R7, must be saved before being used, and
restored at the end (before the next step). Note that if the fast interrupt is programmed
to be level sensitive, the source of the interrupt must be cleared during this phase in
order to de-assert the interrupt source 0.
(with instruction SUB PC, LR, #4 for example). This has the effect of returning from
the interrupt to whatever was being executed before, loading the CPSR with the SPSR
and masking or unmasking the fast interrupt depending on the state saved in the
SPSR.
The “F” bit in SPSR is significant. If it is set, it indicates that the ARM core was just about to mask
FIQ interrupts when the mask instruction was interrupted. Hence when the SPSR is restored, the
interrupted instruction is completed (FIQ is masked).
AT91SAM9263 Preliminary
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