I'd like to state disagreement with the premise that one should naturally
use binary on a microcontroller like a PIC. When the bulk of an application
deals with input and output in decimal (especially scaled values) then it is
often easier to compute in BCD. A relatively simple 'accumulator-based'
library of routines can provide simple routines to do BCD operations. For
example:


    movlw    MYVAL                        ; point to LSDigits of value
    movwf    FSR
    movlw    MYVAL_E-MYVAL    ; length of value
    call         bcd_add

Bob Ammerman
RAm Systems

----- Original Message -----
From: "John Brown" <john.brown@ANU.EDU.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, October 07, 2002 6:58 PM
Subject: Re: [PIC]: BCD time-of-day addition - working solution


> In my case am reading BCD from a Dallas Time and Date chip (ie BCD), and
> have times and dates of particular events that I want to take places
stored
> in EPROM (also in BCD since they are easy to enter and read in hex and
easy
> to compare to the time and date I get from the Dallas chip). Therefore it
> has made things pretty simple to stick with BCD so far.
>
> However some events need to be timed (eg water the grass for 30 minutes
> once it gets dark - ie I don't know the precise starting time so I need to
> calculate the end time).
>
> I contemplated changing everything over to binary, but think I'll give it
a
> go in BCD for now.
>
> Having considered the other replies I came up with the following working
> solution (16f877), derived basically by cut, pasting and modifying Scott
> Dattalo's bcd_add
>
(http://www.piclist.com/techref/microchip/math/add/bcdp2.htm?key=bcd&from=):
>
>
> ;---START--- add the number of minutes in OnTime to TmpMin and increment
> TmpHour if necessary
> ;            The resultant time is put into in ResHour and ResMin
> ;           NOTE: only tested for OnTime < 60 minutes
> ;           NOTE: if the resultant time is between midnight and 1am then
> ResHour will = 24
> ; This routine an adapted version of Scott Dattalo's bcd_add from the PIC
> list web site
> AddMins MOVF    TmpHour,W       ;by default make the resultant hour the
> same as TmpHour
>          MOVWF   ResHour         ;_(it will be incremented later if
necessary)
>          COMF    TmpMin, W       ;add TmpMin and OnTime
>          ADDLW   0x9a+1
>          SUBWF   OnTime, W
>          RLF     ResMin, F
>          BTFSS   STATUS,DC       ;_ if lsn of x + lsn of y < 10 (dec)
>           ADDLW  -0x06           ;__ then remove the extra 6 added above
>          BTFSS   ResMin, 0       ;_ Similarly for the msn
>           ADDLW  -0x60
>          RRF     ResMin, F       ;_restore the carry
>          MOVWF   ResMin
>          MOVLW   0x60            ;see if resultant number of minutes > 59
>          SUBWF   ResMin,W
>          BTFSS   STATUS,C
>          GOTO    aend            ;_ if not, then go to the end of the
routine
>          MOVWF   ResMin          ;_ if it is then the result is the
correct
> number of minutes
>          COMF    TmpHour, W      ;__ and you need to add 1 to TmpHour
(BCD)
>          ADDLW   0x9a+1
>          SUBLW   0x01
>          RLF     ResHour, F
>          BTFSS   STATUS,DC       ;_ if lsn of x + lsn of y < 10 (dec)
>           ADDLW  -0x06           ;__ then remove the extra 6 added above
>          BTFSS   ResHour, 0      ;_ Similarly for the msn
>           ADDLW  -0x60
>          RRF     ResHour, F      ;_ restore the carry
>          MOVWF   ResHour
> aend    RETURN
> ;---End--- add the number of minutes in OnTime to TmpMin and increment
> TmpHour if necessary
>
> John
>
> At 03:55 PM 06/10/2002 +0100, you wrote:
> >This is not a direct solution to your problem but an attempt to rethink
it.
> >
> >Why add in BCD? It seems unnatural on a PIC. If you are using a PIC, then
> >the natural method would be binary addition.
> >
> >I prefer to work in binary and only convert to BCD (or ASCII) when
> >necessary, which is usually for output on a device.
> >
> >--
> >http://www.piclist.com hint: The PICList is archived three different
> >ways.  See http://www.piclist.com/#archives for details.
>
> _______________________________________________________________________
> John Brown
> PhD (Clinical Psychology) candidate
>                                        Email:  john.brown@anu.edu.au
> School of Psychology                  Phone:  (02) 6125-3827
> Room 124, Building 39, Psychology     Fax:    (02) 6125-0499
> The Australian National University    Mobile: 0429 455 504
> ACTON ACT 0200   Web: http://www.anu.edu.au/psychology/staff/BrownJ.htm
>
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