25/5/'98


Hello there everyone,

Quick question for you all (I don't have my data book) on the F84, under
external interrupt control:- Does the PIC complete the current instruction
and increment the PC before entering the interrupt routine? By this i mean:-


INSTRUCTION
DO INTERRUPT
INSTRUCTION
DO INTERRUPT
INSTRUCTION

Or does the PIC use that nasty 6800 interrupt control (Personal opinion),
where the PC is not incremented to the next instruction to perform? (ie, the
processor can remain in the interrupt routine forever) So does the PIC also
then perfrom the current instruction before entering the interrupt?

If this is so, then the PIC can be made to effectively single step. It would
be easier to implement if the PIC also exited the routine before checking
for a re interrupt. If this is so then by holding an interrupt (May need F/F
to the input if it has to be reset) Then the programmer will be able to
single step the PIC trough a routine with just one pin and a small amount of
external hardware. This could be extended to a run until type thing by using
another pin to arm the single step control.
It is just an idea that I have, note that if this is so, then timers and
other interrupts can not run unless they can disarms the single stepper, but
this may cause the code to "Step through" rather than stop at the nominated
point.

Any ideas? Does the PIC work like this?

PS this is the same method used to single step 8051 under software, on other
processors like the Z80 and NSC800 eternal control of the HALT pin is used.

Dennis