SAB80515 Datasheet, PDF (123/270 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller Family

Interrupt System
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being responded to for one of the conditions already mentioned, or if the flag is no longer active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 8-9.
Figure 8-9
Interrupt Response Timing Diagram
Note that if an interrupt of a higher priority level goes active prior to S5P2 in the machine cycle
labeled C3 in figure 8-9, then, in accordance with the above rules, it will be vectored too during C5
and C6 without any instruction for the lower priority routine to be executed.
Thus, the processor acknowledges an interrupt request by executing a hardware-generated LCALL
to the appropriate servicing routine. In some cases it also clears the flag that generated the
interrupt, while in other cases it does not; then this has to be done by the user’s software. The
hardware clears the external interrupt flags IE0 and lE1 only if they were transition-activated. The
hardware-generated LCALL pushes the contents of the program counter onto the stack (but it does
not save the PSW) and reloads the program counter with an address that depends on the source
of the interrupt being vectored too, as shown in the following (table 8-2).
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