C511 datasheet(94/128 Pages) SIEMENS

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Interrupt System

Semiconductor Group

7-7

7.3

How Interrupts are Handled

The interrupt flags are sampled at S5P2 in each machine cycle. The sampled flags are polled during

the following machine cycle. If one of the flags was in a set condition at S5P2 of the preceeding

cycle, the polling cycle will find it and the interrupt system will generate a LCALL to the appropriate

service routine, provided this hardware-generated LCALL is not blocked by any of the following

conditions:

An interrupt of equal or higher priority is already in progress.

The current (polling) cycle is not in the final cycle of the instruction in progress.

The instruction in progress is RETI or any write access to registers IE or IP.

Any of these three conditions will block the generation of the LCALL to the interrupt service routine.

Condition 2 ensures that the instruction in progress is completed before vectoring to any service

routine. Condition 3 ensures that if the instruction in progress is RETI or any write access to

registers IE or IP, then at least one more instruction will be executed before any interrupt is vectored

too; this delay guarantees that changes of the interrupt status can be observed by the CPU.

The polling cycle is repeated with each machine cycle and the values polled are the values that

were present at S5P2 of the previous machine cycle. Note that if any interrupt flag is active but not

being responded to for one of the conditions already mentioned, or if the flag is no longer active

when the blocking condition is removed, the denied interrupt will not be serviced. In other words, the

fact that the interrupt flag was once active but not serviced is not remembered. Every polling cycle

interrogates only the pending interrupt requests.

The polling cycle/LCALL sequence is illustrated in figure 7-2.

Figure 7-2

Interrupt Response Timing Diagram

MCT01859

S5P2

Interrupt

is latched

Interrupts

are polled

Vector Address

Long Call to Interrupt

Routine

Interrupt

C511 Datasheet(PDF) 94 Page - Siemens Semiconductor Group