10.0 Interrupts (Continued)
10.3.1 VIS Execution
When the VIS instruction is executed it activates the arbitra-
tion logic. The arbitration logic generates an even number
between E0 and FE (E0, E2, E4, E6 etc...) depending on
which active interrupt has the highest arbitration ranking at
the time of the 1st cycle of VIS is executed. For example, if
the software trap interrupt is active, FE is generated. If the
external interrupt is active and the software trap interrupt is
not, then FA is generated and so forth. If the only active
interrupt is software trap, than E0 is generated. This number
replaces the lower byte of the PC. The upper byte of the PC
remains unchanged. The new PC is therefore pointing to the
vector of the active interrupt with the highest arbitration
ranking. This vector is read from program memory and
placed into the PC which is now pointed to the 1st instruction
of the service routine of the active interrupt with the highest
arbitration ranking.
Figure 26
illustrates the different steps performed by the VIS
instruction.
Figure 27
shows a flowchart for the VIS instruc-
tion.
The non-maskable interrupt pending flag is cleared by the
RPND (Reset Non-Maskable Pending Bit) instruction (under
certain conditions) and upon RESET.
10131729
FIGURE 26. VIS Operation
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