ST92F124/F150/F250 - INTERRUPTS
ARBITRATION MODES (Cont’d)
End of Interrupt Routine
– If ENCSR is reset, CSR is used instead of ISR,
unless the program returns to another nested
routine.
The iret Interrupt Return instruction executes
the following steps:
The suspended routine thus resumes at the inter-
rupted instruction.
– The Flag register is popped from system stack.
– If ENCSR is set, CSR is popped from system
stack.
Figure 48 contains a simple example, showing that
if the ei instruction is not used in the interrupt
service routines, nested and concurrent modes
are equivalent.
– The PC high byte is popped from system stack.
– The PC low byte is popped from system stack.
Figure 49 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.
– 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.
Figure 49. Complex Example of a Sequence of Interrupt Requests with:
- Nested mode
- IEN set to 1 during the interrupt routine execution
Priority Level of
Interrupt Request
0
INTERRUPT 0 HAS PRIORITY LEVEL 0
INTERRUPT 2 HAS PRIORITY LEVEL 2
INTERRUPT 3 HAS PRIORITY LEVEL 3
INTERRUPT 4 HAS PRIORITY LEVEL 4
INTERRUPT 5 HAS PRIORITY LEVEL 5
INTERRUPT 6 HAS PRIORITY LEVEL 6
INT 0
CPL=0
CPL6 > CPL3:
INT6 pending
1
2
3
INT0
INT 2
INT 2
INT 2
CPL=2
INT6
CPL=2
CPL=2
INT 3
ei
INT2
CPL=3
ei
INT2
INT3
INT4
4
5
6
7
INT 4
CPL=4
INT 4
CPL=4
CPL2 < CPL4:
Serviced just after ei
ei
INT 5
CPL=5
INT 5
CPL=5
ei
INT 6
CPL=6
ei
INT5
MAIN
CPL is set to 7
MAIN
CPL=7
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STMICROELECTRONICS [ ST ]
