Hi Eduardo,

Do not look at PCLATH, that's just a shadow register which is not going to
be changed when you use GOTO. GOTO does hold the 20bit address on your 18F
device but it puts this directly into the real PC -- PCLATH and PCLATU will
be intacted, but anyway, it should jump to the appropriate address. You need
PCLATH for computed goto when you operate with PCL so you store upper bits
to those shadow registers first and then when you move the computed value to
PCL the micro will copy over the upper bits from the PCLATH/PCLATU to the
real PC -- see Harold's example.

Regards,
Tamas


On 6/28/07, Eduardo Garcia <egnascimento@yahoo.com.br> wrote:
>
> Hello Harold!
>
> Thanks a lot your help!
> I did the suggested and the code works fine.
>
> But, I still don't know why Program Counter wasn't with the correct value.
> I
> thought that GOTO instruction copy all 20 bits to PC, but seems to copy
> only
> the PCL contents.
>
> I say it because in simulator, when I do a GOTO 0x400, PCLATH assumes
> 0x0000
> instead 0x0400.
>
> Thanks,
> Eduardo Garcia
>
>
>
> ----- Original Message -----
> From: "Harold Hallikainen" <harold@hallikainen.org>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Wednesday, June 27, 2007 8:25 PM
> Subject: Re: [PIC] Problems with COMPUTED GOTO and PCLATH
>
>
> >
> >> Hi folks,
> >>     I'm with problems when doing a "computed goto" with a PIC18F4680.
> >>
> >>     I've debugged a lot of times and I don't know what's happening.
> >>
> >>     I did a sample program to illustrate my problem:
> >>
> >> The code below works fine because the "computed goto" program area
> don't
> >> change the PCLATH value. PCLATH always is zero.
> >>
> >> -----------------------------------------------------------------------
> >> PROGRAM   CODE 0x0030
> >> Program:
> >> goto Main
> >>
> >>
> >> ANOTHER_AREA CODE 0x80
> >> Main:
> >>  movlw .2
> >>  addwf PCL
> >>  goto Main0
> >>  goto Main2
> >>  goto Main4
> >>
> >> Main0:
> >>  nop
> >>  goto Main
> >>
> >> Main2:
> >>  nop
> >>  goto Main
> >>
> >> Main4:
> >>  nop
> >>  goto Main4
> >>
> --------------------------------------------------------------------------
> >>
> >> However, IF the Main routine is in 0x100 program memory area (or 0x200,
> >> 0x300, and so on), the problem happens and after the addwf a jump to a
> >> 0x0nn
> >> area is performed.
> >>
> >> THIS CODE DOESN'T
> >>
> WORKS--------------------------------------------------------------------=
------------------------------------------
> >> PROGRAM   CODE 0x0030
> >> blProgram:
> >>  goto Main
> >>
> >>
> >> ANOTHER_AREA CODE 0x100
> >> Main:
> >>  movlw .2
> >>  addwf PCL
> >>  goto Main0
> >>  goto Main2
> >>  goto Main4
> >>
> >> Main0:
> >>  nop
> >>  goto Main
> >>
> >> Main2:
> >>  nop
> >>  goto Main
> >>
> >> Main4:
> >>  nop
> >>  goto Main4
> >>
> -------------------------------------------------------------------------=
-------------------------------------------------------------------------
> >>
> >>
> >> I see in simulator that PCLATH value never is changed. So, when I use
> >> ADDWF
> >> PCL, the program jumps to a 0nn relative area insted 1nn.
> >>
> >> "goto" shouldn't change the complete program counter?
> >>
> >> Anybody know what I'm doing wrong??
> >>
> >> Sorry my bad english but I hope you understood me.
> >>
> >> Thanks,
> >> Eduardo Garcia
> >>
> >
> >
> > Your English is fine!
> >
> > The goto changes the full program counter, but the addwf PCL ONLY
> changes
> > PCL.
> >
> > I'd do something like this:
> >
> >
> >     movlw high(JumpTable)  ; set pclath to start of jump table
> >     movwf PCLATH
> >     RLFNC state,w          ; get state*2 since each bra takes 2
> addresses
> >     addwf low(JumpTable),w ; add in low half of start of table
> >     skpnc
> >     incf  PLCATH,f         ; add carry if needed
> >     movwf PCL              ; jump into table
> > JumpTable
> >     bra main0
> >     bra main1
> >     bra main2
> >
> > Note the use of bra instead of goto. If you use goto, you'll have to
> > allocate 4 addresses per state. Using bra lets you do a RLFNC to
> multiply
> > by two as you get the state into w.
> >
> > Hope that helps!
> >
> > Harold
> > --
> > FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
> > opportunities available!
> > --
> > http://www.piclist.com PIC/SX FAQ & list archive
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>
>
>
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