Post by thread:Inttrupt routine & another inttrupt?

On Wed, 21 Oct 1998 17:34:22 +0200 Radboud Verberne <spam_OUTr.verberneTakeThisOuThsbos.nl> writes: >Just a question: What happens if your program is doing the inttrupt >service routine and another Intflag getting set? Is the inttrupt directly >detected by the PIC after the retfie instuction? Yes... unless your "main ISR" has not yet dealt with that interrupt. My typical interrupt handler has a bunch of btfsz instructions polling the appropriate interrupt flag bits followed by calls of the handler for that device. The end of the routine has the ertfie. You COULD enable interrupts in the ISR and allow interrupts to interrupt interrupts, but the limited stack depth in the PIC is sure to cause problems. There are also reentrancy problems in the ISR code. I'm sure that my code often gets an interrupt set after the ISR has already polled that device, causing an immediate interrupt after the retfie. This could be avoided by having the ISR loop 'til none of the interrupt flags are set, possibly speeding interrupt response time since you'd only have to save and restore context once. I haven't had to resort to this (yet), though. Harold Harold Hallikainen .....haroldKILLspam@spam@hallikainen.com Hallikainen & Friends, Inc. See the FCC Rules at http://hallikainen.com/FccRules and comments filed in LPFM proceeding at http://hallikainen.com/lpfm ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com or call Juno at (800) 654-JUNO [654-5866]

part 0 3085 bytes I've done this on some occasions; the PIC's interrupt hardware is extremely simple, so there's no complex "faking" as can sometimes be needed on, e.g., an 8x51. Basically you need to consider that nesting ISR's means two things: (1) The stack depth of the nesting ISR is in addition to that of the current ISR. (2) Anything the nesting ISR does to registers may bash the underlying ISR; either the nesting ISR has to avoid any instructions that may affect flags or W, or else you need seperate file registers to save W and F for the two interrupts. Generally, then, the only time that it makes sense to have ISR's nest is if there's a very simple routine that is allowed to execute during a longer and more complicated one. For example, suppose you have a routine which you would like to execute precisely once every 5000 clock cycles, and which may take over 1000 cycles to execute. Since the RTCC's prescalar won't allow timing durations of that sort, and since writing to the RTCC clears the prescalar (BOO HISS!) the only option is to use a 1:1 prescalar and deal with the interrupt every 256 cycles (once every 20 interrupts we perform the "real" ISR and jog the RTCC a bit). ISR: decfsz Counter,f retfie bsf GIE movwf SaveW swapf STATUS,F movwf SaveStat movlw 123 ; Jog count by 120; add 3 to dea l ; with PIC's synchronizing delay s addwf RTCC,f movlw 20 addwf Counter,f ... real meat of ISR goes here swapf SaveW,f swapf SaveStat,w movwf STATUS swapf SaveW,w retfie Note that interrupts may be safely nested at any point after the third instruction; unless twenty interrupts occur before the main ISR is done the nested interrupts won't disrupt anything. By the way, when doing nested interrupt stuff like this, it's often useful to use INCFSZ/DECFSZ in situations where the skip-on-zero is not needed (provided it won't cause trouble), since unlike INCF/DECF, they don't set flags. Also, using BSF and BCF instructions on the RTCC may be somewhat handy. For example, if it's desired to have the ISR execute every 135 cycles this may be accomplished by bsf RTCC,7 ; Skips ahead 128 counts, but drops 3 decfsz RTCC,f ; Drops back a count, plus 3 more While not nearly as clear as: movlw 259-135 addwf RTCC,f the approach with "bsf" and "decfsz" has the decided advantage that it does not affect any flags or W. In short, nesting ISR's is fine provided that the nested ISR's tread very lightly. It takes a bit of getting used to, but allows for some useful techniques.

Hi, Yeah. That's why one can enter into an endless loop when forgetting to reset the appropriate IF. Imre On Wed, 21 Oct 1998, Radboud Verberne wrote: {Quote hidden}> Hi, > > Just a question: What happens if your program is doing the inttrupt service > routine and another Intflag getting set? Is the inttrupt directly detected > by the PIC after the retfie instuction? > > Greetz, > Radboud Verberne. > ICQ : 918640 (I Seek You) > HAM : PE1RUH > > S-Mail: Eerste Vijverstraat 6 > 5258 HR Berlicum, The Netherlands > Phone : +31-73-503-4733 > >