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ST92141 - INTERRUPTS

ARBITRATION MODES (Cont’d)

Example 2

In the second example, (more complex, Figure

18), each interrupt service routine sets Interrupt

Enable with the ei instruction at the beginning of

the routine. Placed here, it minimizes response

time for requests with a higher priority than the one

being serviced.

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-

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.

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 iret of 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 18. Complex Example of a Sequence of Interrupt Requests with:

- Concurrent mode selected

- IEN set to 1 during interrupt service routine execution

MAIN

INT 5

INT 2

INT 3

INT 4

INT 5

INT 4

INT 3

INT 2

CPL is set to 7

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 3

CPL = 7

INT 5

CPL = 7

MAIN

Priority Level of

Interrupt Request

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