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S3F80P5_UM_ REV1.00
FAST INTERRUPT PROCESSING (Continued)
Two other system registers support fast interrupt processing:
•
The instruction pointer (IP) contains the starting address of the service routine (and is later used to swap the
program counter values), and
•
When a fast interrupt occurs, the contents of the FLAGS register are stored in an unmapped, dedicated
register called FLAGS' ("FLAGS prime").
For the S3F80P5 microcontroller, the service routine for any one of the seven interrupt levels:
IRQ0−IRQ7(IRQ5 is reserved for S3F80P5), can be selected for fast interrupt processing.
Procedure for Initiating Fast Interrupt
To initiate fast interrupt processing, follow these steps:
1. Load the start address of the service routine into the instruction pointer (IP).
2. Load the interrupt level number (IRQn) into the fast interrupt selection field (SYM.4−SYM.2)
3. Write a "1" to the fast interrupt enable bit in the SYM register.
Fast Interrupt Service Routine
When an interrupt occurs in the level selected for fast interrupt processing, the following events occur:
1. The contents of the instruction pointer and the PC are swapped.
2. The FLAG register values are written to the FLAGS' ("FLAGS prime") register.
3. The fast interrupt status bit in the FLAGS register is set.
4. The interrupt is serviced.
5. Assuming that the fast interrupt status bit is set, when the fast interrupt service routine ends, the instruction
pointer and PC values are swapped back.
6. The content of FLAGS' ("FLAGS prime") is copied automatically back to the FLAGS register.
7. The fast interrupt status bit in FLAGS is cleared automatically.
Programming Guidelines
Remember that the only way to enable/disable a fast interrupt is to set/clear the fast interrupt enable bit in the
SYM register, SYM.1. Executing an EI or DI instruction globally enables or disables all interrupt processing,
including fast interrupts. If you use fast interrupts, remember to load the IP with a new start address when the fast
interrupt service routine ends.
NOTE
INTERRUPT STRUCTURE
5-17
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