ST92196A datasheet(50/268 Pages) STMICROELECTRONICS

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INTERRUPTS

ARBITRATION MODES (Cont’d)

End of Interrupt Routine

The iret Interrupt Return instruction executes

the following steps:

– The Flag register is popped from system stack.

– If ENCSR is set, CSR is popped from system

stack.

– The PC high byte is popped from system stack.

– The PC low byte is popped from system stack.

– All unmasked Interrupts are enabled by setting

the CICR.IEN bit.

– The priority level of the interrupted routine is

popped from the special register (NICR) and

copied into CPL.

– If ENCSR is reset, CSR is used instead of ISR,

unless the program returns to another nested

routine.

The suspended routine thus resumes at the inter-

rupted instruction.

Figure 21 contains a simple example, showing that

if the ei instruction is not used in the interrupt

service routines, nested and concurrent modes

are equivalent.

Figure 22 contains a more complex example

showing how nested mode allows nested interrupt

processing (enabled inside the interrupt service

routinesi using the ei instruction) according to

their priority level.

Figure 22. Complex Example of a Sequence of Interrupt Requests with:

- Nested mode

- IEN set to 1 during the interrupt routine execution

INT 2

INT 3

CPL=3

INT 0

CPL=0

INT6

MAIN

INT 5

INT 4

INT0

INT4

INT3

INT2

CPL is set to 7

CPL=5

CPL=4

CPL=2

CPL=7

INTERRUPT 2 HAS PRIORITY LEVEL 2

INTERRUPT 3 HAS PRIORITY LEVEL 3

INTERRUPT 4 HAS PRIORITY LEVEL 4

INTERRUPT 5 HAS PRIORITY LEVEL 5

INT 2

INT 4

CPL=2

CPL=4

INT 5

CPL=5

MAIN

INT 2

CPL=2

INT 6

CPL=6

INT5

INT2

INTERRUPT 6 HAS PRIORITY LEVEL 6

INTERRUPT 0 HAS PRIORITY LEVEL 0

CPL6 > CPL3:

INT6 pending

CPL2 < CPL4:

Serviced just after ei

Priority Level of

Interrupt Request

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