Hi again David,

I just tried this and it seems as though the method I used is OK. In MPLAB,
I set the zero bit in STATUS and then executed a MOVF STATUS,W. The zero
bit was clear in STATUS after the MOVF but the W register held the
unmodified value of STATUS.

Do you see any further problems with it?

Incidentally, is there a way in MPLAB to force the unwritable bits of
STATUS (TO and PD) to particular values? That is, can we simulate a WDT
timeout or wakeup from SLEEP without having to actually place code in the
file to cause those events?

Thanks again,

Sean

At 04:31 PM 1/12/2003 +1000, you wrote:
>Hi Sean,
>I took a really quick peek at the code and noticed that your ISR
>save and restore possibly has a bug in it. When you get the
>status register, you are affecting the zero flag just by reading it
>whereas a swap doesn't. I would have thought that the status bits
>would not change until *after* the MOVF instruction in which case
>STATUS gets cleared anyway in the next line but Microchip seem
>to recommend doing it the following way: (Taken from my code)
>
>        org     4               ;interrupt vector
>        movwf   save_w          ;save contents of working register
>        swapf   status,w        ;get current status into w
>        clrf    status          ;clear all status & ram bank bits
>        movwf   save_s          ;and save it for interrupt restore
>        movf    pclath,w        ;get current code page address
>        movwf   save_p          ;and save it for interrupt restore
>        clrf    pclath          ;select code page 0
>        movf    fsr,w           ;get current fsr position
>        movwf   save_f          ;and save it for interrupt restore
>        ...
>        ...
>        ...
>        movf    save_f,w        ;get saved fsr
>        movwf   fsr             ;& restore it
>        movf    save_p,w        ;get the previous code bank
>        movwf   pclath          ;and restore it
>        swapf   save_s,w        ;get back the status bits
>        movwf   status          ;put into the status register
>        swapf   save_w,f        ;swap the high and low nibbles
>        swapf   save_w,w        ;of the saved w register back into w reg
>        retfie                  ;then return from interrupt
>
>BTW, I like the clock. It's nice to make something that you use everyday.
>I made a simple shower/game timer for my kids one day over Christmas.
>They use it all the time. http://www.users.on.net/avd/pic/index.html
>David...
>
>--
>http://www.piclist.com hint: PICList Posts must start with ONE topic:
>[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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