Post by thread:Erratic PC increment through interrupt

Does anyone know, if there is danger of erratic PC increment caused by an interrupt during a MOVWF PCL ? The application note AN556, concerning table reads / computed GOTO on a PIC 16Cxx device, says, that one has to disable interrupts to prevent the computed GOTO from jumping one instruction too far. The data book (at least for PIC 17C42) says, that increment of PC is inhibited after GOTO, CALL, RETURN... and MOVWF PCL. So then, there should be no danger? Or even worse, an interreupt during a normal GOTO could also cause a false jump ?!? I would like to hear, if anyone has tested for this problem. TIA Wolfram +-----------------------------------------------+ ! Wolfram Liebchen, spam_OUTliebchenTakeThisOuTipserv.ffo.fgan.de ! ! Forschungsinstitut fuer Optik ! ! Schloss Kressbach ! ! D-72072 Tuebingen ! ! Tel: +49 (0)7071 / 709-158 ! ! Fax: +49 (0)7071 / 709-270 (G3 / G4) ! +-----------------------------------------------+

>Does anyone know, if there is danger of erratic PC increment >caused by an interrupt during a MOVWF PCL ? >The application note AN556, concerning table reads / computed GOTO >on a PIC 16Cxx device, says, that one has to disable interrupts to >prevent the computed GOTO from jumping one instruction too far. > > >Wolfram Wolfram, my ownly comment would be that interrupts may be disabled during a "movwf PCL", but I believe the command that is normally used in tables/computed jumps is "addwf PCL,f" (which is not in your list of interrupt disabling commands). I think I have lucked out in my applications by doing table reads _before_ interrupts are enabled and inside interrupt handlers (in which interrupts are disabled). I will definitely add this to my list of "things to avoid". Myke Myke "We're Starfleet officers, weird is part of the job." Capt. Catherine Janeway

Wolfram Liebchen wrote: > > Does anyone know, if there is danger of erratic PC increment > caused by an interrupt during a MOVWF PCL ? > The application note AN556, concerning table reads / computed GOTO > on a PIC 16Cxx device, says, that one has to disable interrupts to > prevent the computed GOTO from jumping one instruction too far. > > The data book (at least for PIC 17C42) says, that increment of PC > is inhibited after GOTO, CALL, RETURN... and MOVWF PCL. So then, there > should be no danger? > Or even worse, an interreupt during a normal GOTO could also cause a > false jump ?!? > > I would like to hear, if anyone has tested for this problem. > The information in AN556 is incorrect. You do not need to disable interrupts in computed GOTOs. Scott P.S. Andy W., I just checked your PIC question 5. It still references AN576.