ST90158 - INTERRUPTS
ARBITRATION MODES (Cont’d)
Example 2
In the second example, (more complex, Figure
22), each interrupt service routine sets Interrupt
Enable with the eiinstruction at the beginning of
the routine. Placed here, it minimizes response
time for requests with a higher priority than the one
being serviced.
ice routines being executed in the opposite order
of their priority.
It is therefore recommended to avoid inserting
the ei instruction in the interrupt service rou-
tine in Concurrent mode. Use the ei instruc-
tion only in nested mode.
The level 2 interrupt routine (with the highest prior-
ity) will be acknowledged first, then, when the ei
instruction is executed, it will be interrupted by the
level 3 interrupt routine, which itself will be inter-
rupted by the level 4 interrupt routine. When the
level 4 interrupt routine is completed, the level 3 in-
terrupt routine resumes and finally the level 2 inter-
rupt routine. This results in the three interrupt serv-
WARNING: If, in Concurrent Mode, interrupts are
nested (by executing ei in an interrupt service
routine), make sure that either ENCSR is set or
CSR=ISR, otherwise the iretof the innermost in-
terrupt will make the CPU use CSR instead of ISR
before the outermost interrupt service routine is
terminated, thus making the outermost routine fail.
Figure 22. Complex Example of a Sequence of Interrupt Requests with:
- Concurrent mode selected
- IEN set to 1 during interrupt service routine execution
0
Priority Level of
Interrupt Request
INTERRUPT 2 HAS PRIORITY LEVEL 2
INTERRUPT 3 HAS PRIORITY LEVEL 3
INTERRUPT 4 HAS PRIORITY LEVEL 4
INTERRUPT 5 HAS PRIORITY LEVEL 5
1
2
3
4
5
6
7
INT 2
INT 2
CPL = 7
CPL = 7
INT 3
CPL = 7
INT 3
CPL = 7
ei
INT 2
INT 3
INT 4
ei
ei
INT 4
CPL = 7
INT 5
INT 5
CPL = 7
CPL = 7
ei
ei
INT 5
MAIN
MAIN
CPL is set to 7
CPL = 7
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ETC [ ETC ]
