In a  16C63 program, I am in a tight loop waiting for an input from portA.
Whilst this loop is running, characters are received by the hardware uart
using the receiver interrupts.

Now some of the time it works just fine but every now and again the program
leaves the tight loop and
calls 'run_sequence' when it shouldn't  This happens both with the device
and the emulator. A scope on PortA shows
no noise and a permanent low level, it is a switch contact only a few cms
from the device anyhow.
The ISR doesn't  read or write PortA.

flash_on_trigger        CLRWDT
                BTFSS FLAGS,FLASH_EN
                GOTO fot_end
                BTFSS PORTA,TRIG        ;I have tried forcing PORTA,TRIG to a ha
rd low
                GOTO flash_on_trigger
                CALL run_sequence       ; but when my program comes back from th
e isr it
sometimes ends up here !!!!!!!!!!!!!!!!!!!!!!
                RETURN

; I haven't shown my isr


run sequence          BSF PORTB,LED             ;Should not arrive here without
TRIG
going high!
                NOP
                NOP
                BCF PORTB,LED
                NOP
                NOP
                RETURN



Is this a known problem? i.e. if the interrupt occurs during the GOTO
flash_on_trigger will the PC be incremented
to point to CALL run_sequence instead of looping to label flash_on_trigger.

Can anyone help please - before I pull my remaining hair out.   :-(

Regards,
Gerry Cox
gcox@dek.com