Post by thread:[PIC]:code problem

Hi, This my second program on a PIC. I'm using a 16F877 and having some trouble getting the right value into w. Tthis starts with a value already in w such as 'a'. If w = 'a', I want to replace it with 'x' If w != 'a' leave as is. The trouble is the last instruction, movlw _w puts 0x21 into w. I set a watch on w and _w and all looks good until the last instruction. This is probably something elementary but it's got me stuck. Appreciatte any help. movwf _w ;save w xorlw 0x61 ;compare w to 'a' btfss status,2 goto send movlw 0x78 ; x movwf _w ;restore w with orig or new char send movlw _w -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 01:38 PM 8/12/00 -0700, you wrote: {Quote hidden}>Hi, >This my second program on a PIC. I'm using a 16F877 and having >some trouble getting the right value into w. > >Tthis starts with a value already in w such as 'a'. >If w = 'a', I want to replace it with 'x' >If w != 'a' leave as is. >The trouble is the last instruction, movlw _w puts 0x21 into w. >I set a watch on w and _w and all looks good until the last instruction. >This is probably something elementary but it's got me stuck. >Appreciatte any help. > > movwf _w ;save w > xorlw 0x61 ;compare w to 'a' > btfss status,2 > goto send > movlw 0x78 ; x > movwf _w ;restore w with orig or new char > >send > movlw _w > As written here, "movlw" command means "move literal to w", where literal = an 8-bit constant. 0x21 is probably the address of your variable _w. BTW, just a hint - it usually makes code more readable if you write "btfss status,Z" rather than "btfss status,2" Also, some people write "movf _w,0" and "movf _w,1" whereas "movf _w,w" and "movf _w,f" are much more readable. best regards, - danM -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

The last instruction could be: movfw _W in order to get the CONTENTS of _W register into W register. instead of movlw _W -----Mensaje original----- De: Dan Michaels <spam_OUToricomTakeThisOuTLYNX.SNI.NET> Para: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU <PICLISTKILLspamMITVMA.MIT.EDU> Fecha: Sabado 12 de Agosto de 2000 02:16 PM Asunto: Re: [PIC]:code problem {Quote hidden}>At 01:38 PM 8/12/00 -0700, you wrote: >>Hi, >>This my second program on a PIC. I'm using a 16F877 and having >>some trouble getting the right value into w. >> >>Tthis starts with a value already in w such as 'a'. >>If w = 'a', I want to replace it with 'x' >>If w != 'a' leave as is. >>The trouble is the last instruction, movlw _w puts 0x21 into w. >>I set a watch on w and _w and all looks good until the last instruction. >>This is probably something elementary but it's got me stuck. >>Appreciatte any help. >> >> movwf _w ;save w >> xorlw 0x61 ;compare w to 'a' >> btfss status,2 >> goto send >> movlw 0x78 ; x >> movwf _w ;restore w with orig or new char >> >>send >> movlw _w >> > >As written here, "movlw" command means "move literal to w", >where literal = an 8-bit constant. 0x21 is probably the >address of your variable _w. > >BTW, just a hint - it usually makes code more readable if >you write > >"btfss status,Z" rather than "btfss status,2" > >Also, some people write > >"movf _w,0" and "movf _w,1" whereas > >"movf _w,w" and "movf _w,f" are much more readable. > >best regards, >- danM > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 01:38 PM 8/12/00 -0700, you wrote: {Quote hidden}>Hi, >This my second program on a PIC. I'm using a 16F877 and having >some trouble getting the right value into w. > >Tthis starts with a value already in w such as 'a'. >If w = 'a', I want to replace it with 'x' >If w != 'a' leave as is. >The trouble is the last instruction, movlw _w puts 0x21 into w. >I set a watch on w and _w and all looks good until the last instruction. >This is probably something elementary but it's got me stuck. >Appreciatte any help. > > movwf _w ;save w > xorlw 0x61 ;compare w to 'a' > btfss status,2 > goto send > movlw 0x78 ; x > movwf _w ;restore w with orig or new char > >send > movlw _w > BTW, another hint related to choosing names for variables, and trying to debug code that is not working properly.... Using _w for a variable name can cause some real problems, because w is the most used register in a PIC. Therefore, in a program with 100 lines, you might end up with _w or w in almost every line. If you accidentally type _w instead of w, or vice versa, somewhere in the program, it might take months to figure out what the problem is. Better to use something like temp_w or sav_w - variable names that are more distinctive. best regards, - danM -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Sandy, It can be done also like this: xorlw 'a' ;compare to 'a', w' = w ^ 'a' btfss STATUS, Z movlw 'x'^'a' ;if not equal, w = 'x' ^ 'a' xorlw 'a' ;if w was equal to 'a', restore it ;if w was unequal to 'a', ; w = 'x' ^ 'a' ^ 'a'= 'x' Cheers, Nikolai ---- Original Message ---- From: Sandy Phelps Sent: Saturday, August 12, 2000 23:38:33 To: .....PICLISTKILLspam.....MITVMA.MIT.EDU Subj: [PIC]:code problem {Quote hidden}> Hi, > This my second program on a PIC. I'm using a 16F877 and having > some trouble getting the right value into w. > Tthis starts with a value already in w such as 'a'. > If w = 'a', I want to replace it with 'x' > If w != 'a' leave as is. > The trouble is the last instruction, movlw _w puts 0x21 into w. > I set a watch on w and _w and all looks good until the last instruction. > This is probably something elementary but it's got me stuck. > Appreciatte any help. > movwf _w ;save w > xorlw 0x61 ;compare w to 'a' > btfss status,2 > goto send > movlw 0x78 ; x > movwf _w ;restore w with orig or new char > send > movlw _w > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

movfw is one of the built in 'macro' instructions, that is an instruction that can also be written in a more general form, in this case movf <r>,w. The seemingly useless movf <r>,f (move f to f) can be used to test a register for zero without affecting w, though I believe it is rarely used. There is another usefull set, the skip instructions, these expand out to bit test instructions acting on the status register, for example skpnz skip next instruction if zero flag not set. There are set-bit and clear-bit instructions. A handy trick to remember is that an exclusive-or is reversable so you can write: xorlw 0x61 ;compare w to 'a' btfsc status,Z movlw 0x61 ^ 0x78; that's 0x19 but writing the expression is clearer xorlw 0x61 ;put w back how it was You may find yourself able to eliminate many gotos that way. This can make code smaller and avoids the need for a lot of labels. It also makes the code isochronous, it always takes the same time to execute regardless of results. {Original Message removed}