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S3C9228/P9228 INTERRUPT STRUCTURE
5-3
INTERRUPT SOURCE SERVICE SEQUENCE
The interrupt request polling and servicing sequence is as follows:
1. A source generates an interrupt request by setting the interrupt request pending bit to "1".
2. The CPU generates an interrupt acknowledge signal.
3. The service routine starts and the source's pending flag is cleared to "0" by software.
4. Interrupt priority must be determined by software polling method.
INTERRUPT SERVICE ROUTINES
Before an interrupt request can be serviced, the following conditions must be met:
— Interrupt processing must be enabled (EI, SYM.3 = "1")
— Interrupt must be enabled at the interrupt's source (peripheral control register)
If all of the above conditions are met, the interrupt request is acknowledged at the end of the instruction cycle.
The CPU then initiates an interrupt machine cycle that completes the following processing sequence:
1. Reset (clear to "0") the global interrupt enable bit in the SYM register (DI, SYM.3 = "0")
to disable all subsequent interrupts.
2. Save the program counter and status flags to stack.
3. Branch to the interrupt vector to fetch the service routine's address.
4. Pass control to the interrupt service routine.
When the interrupt service routine is completed, an Interrupt Return instruction (IRET) occurs. The IRET restores
the PC and status flags and sets SYM.3 to "1"(EI), allowing the CPU to process the next interrupt request.
GENERATING INTERRUPT VECTOR ADDRESSES
The interrupt vector area in the ROM contains the address of the interrupt service routine. Vectored interrupt
processing follows this sequence:
1. Push the program counter's low-byte value to stack.
2. Push the program counter's high-byte value to stack.
3. Push the FLAGS register values to stack.
4. Fetch the service routine's high-byte address from the vector address 0000H.
5. Fetch the service routine's low-byte address from the vector address 0001H.
6. Branch to the service routine specified by the 16-bit vector address.