ST92F120 datasheet(75/320 Pages) STMICROELECTRONICS

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ST92F120 - INTERRUPTS

ARBITRATION MODES (Cont’d)

re-enabled, they will be acknowledged regardless

of the interrupt service routine’s priority. This may

cause undesirable interrupt response sequences.

Examples

In the following two examples, three interrupt re-

quests with different priority levels (2, 3 & 4) occur

simultaneously during the interrupt 5 service rou-

tine.

Example 1

In the first example, (simplest case, Figure 34) the

instruction is not used within the interrupt serv-

ice routines. This means that no new interrupt can

be serviced in the middle of the current one. The

interrupt routines will thus be serviced one after

another, in the order of their priority, until the main

program eventually resumes.

Figure 34. Simple Example of a Sequence of Interrupt Requests with:

- Concurrent mode selected and

- IEN unchanged by the interrupt routines

Priority Level of

MAIN

INT 5

INT 2

INT 3

INT 4

MAIN

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

Interrupt Request

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