Post by thread:[PIC]: Spot the Bug! AKA table reads

Hi, I'm building a project with the 16F877 chip interfaced to a Hitachi type LCD. I have plenty of LCD screens to display (much more than 256 chars) and so I want to implement them all at the top of the program memory. However when I do that I run in to table read errors, and my program crashes. I've read the Microchip app notes on implementing a safe table read in high memory, and used the method they say, but it still crashes! I just can't figure it out. The text table is at ORG 1800h and just has 4 RETLW instructions, to print out the letters A, B, C, D as a test. At the start of the troublesome routine I print a ">" to indicate I'm in the display_message routine, and a "<" at the end to show I'm finished. The program prints the ">" on the LCD and then crashes. Here's extracts of my code. Can anyone spot the bug? TABLE_ADDR EQU 0x1800 ;-----------------------------------------------------------------------; ; display_welcome ; ;-----------------------------------------------------------------------; display_message movlw 0 movf offset movlw ">" call send_ascii movlw LOW TABLE addwf offset movlw HIGH TABLE btfsc status,c addlw 1 movwf PCLATH movf offset,w call TABLE call send_ascii movlw "<" call send_ascii return ;-----------------------------------------------------------------------; ; Messages ; ;-----------------------------------------------------------------------; ORG TABLE_ADDR TABLE movwf PCL ; called with displacement in W retlw "A" retlw "B" retlw "C" retlw "D" Any help appreciated! Paul -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use spam_OUTlistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

Hi, [ Apologies if this has been sent twice! ] I'm building a project with the 16F877 chip interfaced to a Hitachi type LCD. I have plenty of LCD screens to display (much more than 256 chars) and so I want to implement them all at the top of the program memory. However when I do that I run in to table read errors, and my program crashes. I've read the Microchip app notes on implementing a safe table read in high memory, and used the method they say, but it still crashes! I just can't figure it out. The text table is at ORG 1800h and just has 4 RETLW instructions, to print out the letters A, B, C, D as a test. At the start of the troublesome routine I print a ">" to indicate I'm in the display_message routine, and a "<" at the end to show I'm finished. The program prints the ">" on the LCD and then crashes. Can anyone spot the bug? Here's extracts of my code. The routines for setting up the ports, and initialising the LCD are not shown. TABLE_ADDR EQU 0x1800 ;-----------------------------------------------------------------------; ; display_welcome ; ;-----------------------------------------------------------------------; display_message movlw 0 movf offset movlw ">" call send_ascii movlw LOW TABLE addwf offset movlw HIGH TABLE btfsc status,c addlw 1 movwf PCLATH movf offset,w call TABLE call send_ascii movlw "<" call send_ascii return ;-----------------------------------------------------------------------; ; Messages ; ;-----------------------------------------------------------------------; ORG TABLE_ADDR TABLE movwf PCL ; called with displacement in W retlw "A" retlw "B" retlw "C" retlw "D" Any help appreciated! Paul -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use .....listservKILLspam@spam@mitvma.mit.edu?body=SET%20PICList%20DIGEST

Just possibly, with your:- display_message movlw 0 movf offset the movf offset should be movf offset,w ? Otherwise you're just moving offset to itself and possibly running out the top end of the table when you do the lookup? I've found that it pays before you call the table lookup routine to make sure the w reg is within allowable limits. Hope this helps Richard P Hi, I'm building a project with the 16F877 chip interfaced to a Hitachi type LCD. I have plenty of LCD screens to display (much more than 256 chars) and so I want to implement them all at the top of the program memory. However when I do that I run in to table read errors, and my program crashes. I've read the Microchip app notes on implementing a safe table read in high memory, and used the method they say, but it still crashes! I just can't figure it out. The text table is at ORG 1800h and just has 4 RETLW instructions, to print out the letters A, B, C, D as a test. At the start of the troublesome routine I print a ">" to indicate I'm in the display_message routine, and a "<" at the end to show I'm finished. The program prints the ">" on the LCD and then crashes. Here's extracts of my code. Can anyone spot the bug? TABLE_ADDR EQU 0x1800 ;-----------------------------------------------------------------------; ; display_welcome ; ;-----------------------------------------------------------------------; display_message movlw 0 movf offset movlw ">" call send_ascii movlw LOW TABLE addwf offset movlw HIGH TABLE btfsc status,c addlw 1 movwf PCLATH movf offset,w call TABLE call send_ascii movlw "<" call send_ascii return ;-----------------------------------------------------------------------; ; Messages ; ;-----------------------------------------------------------------------; ORG TABLE_ADDR TABLE movwf PCL ; called with displacement in W retlw "A" retlw "B" retlw "C" retlw "D" Any help appreciated! Paul -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use .....listservKILLspam.....mitvma.mit.edu?body=SET%20PICList%20DIGEST

Paul: I do this a little differently: First, get the offset into w, then call the table, then add the offset. (Be sure to include the destination (f) in the addwf instruction.) TABLE > addwf PCL,f ; called with displacement in W > retlw "A" > retlw "B" > retlw "C" > retlw "D" The thing to remember is that when you do the "addwf PCL,f", the computed address will include the PCLATH, which in your case, will have to be 0x18. (The contents of w can only range to FF, so the rest of the address has to come from PCLATH. I have done a very similar thing as you describe (lots of text messages to a Hitatchi LCD), and sometimes have found it useful to have a separate table just to return the appropriate PCLATH value prior to calling the message table. Hope this helps. Foster > {Original Message removed}

Paul Reilly wrote: {Quote hidden}> Hi, > > I'm building a project with the 16F877 chip interfaced to a Hitachi type > LCD. I have plenty of LCD screens to display (much more than 256 > chars) and so I want to implement them all at the top of the program > memory. > > However when I do that I run in to table read errors, and my program > crashes. I've read the Microchip app notes on implementing a safe table > read in high memory, and used the method they say, but it still crashes! > I just can't figure it out. > > The text table is at ORG 1800h and just has 4 RETLW instructions, to > print out the letters A, B, C, D as a test. At the start of the > troublesome routine I print a ">" to indicate I'm in the display_message > routine, and a "<" at the end to show I'm finished. The program prints the > ">" on the LCD and then crashes. > > Here's extracts of my code. Can anyone spot the bug? > > TABLE_ADDR EQU 0x1800 > > ;-----------------------------------------------------------------------; > ; display_welcome ; > ;-----------------------------------------------------------------------; > > display_message > movlw 0 > movf offset < What are you moving OFFSET to?, or is this > a test of the register? Or did you mean MOVWF? > > movlw ">" > call send_ascii > > movlw LOW TABLE > addwf offset > movlw HIGH TABLE > btfsc status,c > addlw 1 > movwf PCLATH > movf offset,w > > call TABLE > call send_ascii > > movlw "<" > call send_ascii > > return > > ;-----------------------------------------------------------------------; > ; Messages ; > ;-----------------------------------------------------------------------; > > ORG TABLE_ADDR > > TABLE > movwf PCL ; called with displacement in W > retlw "A" > retlw "B" > retlw "C" > retlw "D" > > Any help appreciated! > > Paul > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > use EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST -- * | __O Thomas C. Sefranek tcsspam_OUTcmcorp.com |_-\<,_ Amateur Radio Operator: WA1RHP (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz hamradio.cmcorp.com/inventory/Inventory.html http://www.harvardrepeater.org -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use @spam@listservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST

> Here's extracts of my code. Can anyone spot the bug? Yes, I can. However I don't believe in helping those who haven't even done the simple little things to help themselves. There is not a single comment in the first section of code below. Go back and force yourself to explain what you think each instruction is doing, and there's a reasonable chance you'll figure out the problem on your own. Another suggestion is to look over the instruction set again. You are doing some things the hard way. {Quote hidden}> display_message > movlw 0 > movf offset > > movlw ">" > call send_ascii > > movlw LOW TABLE > addwf offset > movlw HIGH TABLE > btfsc status,c > addlw 1 > movwf PCLATH > movf offset,w > > call TABLE > call send_ascii > > movlw "<" > call send_ascii > > return > TABLE > movwf PCL ; called with displacement in W > retlw "A" > retlw "B" > retlw "C" > retlw "D" ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, KILLspamolinKILLspamcognivis.com, http://www.cognivis.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Olin's personality doesn't always rub me the right way, BUT... ...he IS right, I think, in that commenting code helps to find bugs. Now, that doesn't mean that other people can't help out and point out the obvious bug, I just wanted to try to get past the... personality issues... and point out that going over this code and commenting it would be a good way to find the problem. Especially if you did the commenting by looking up the instructions and reading their function. --- James Newton (PICList Admin #3) spamBeGonejamesnewtonspamBeGonepiclist.com 1-619-652-0593 PIC/PICList FAQ: http://www.piclist.com or .org {Original Message removed}

On Thu, 12 Oct 2000, Paul Reilly wrote: > > Here's extracts of my code. Can anyone spot the bug? I see three. {Quote hidden}> > > > TABLE_ADDR EQU 0x1800 > > ;-----------------------------------------------------------------------; > ; display_welcome ; > ;-----------------------------------------------------------------------; > > > display_message > movlw 0 > movf offset clrf offset > > movlw ">" > call send_ascii > > movlw LOW TABLE > addwf offset ;since offset may've changed in send_ascii, it's safer to do this: movf offset,w andlw 3 ;There are only 4 entries ;so make sure the upper bits are cleared addlw LOW(TABLE) movwf offset {Quote hidden}> movlw HIGH TABLE > btfsc status,c > addlw 1 > movwf PCLATH > movf offset,w > > call TABLE > call send_ascii > > movlw "<" > call send_ascii > > return > > ;-----------------------------------------------------------------------; > ; Messages ; > ;-----------------------------------------------------------------------; > > ORG TABLE_ADDR > > TABLE > movwf PCL ; called with displacement in W > retlw "A" > retlw "B" > retlw "C" > retlw "D" > but I'd probably just implement this so: fetchchar: movwf offset movlw HIGH(TABLE) movwf PCLATH ;okay now, this get's used only at the branch movf offset,w andlw 3 ;only 4 table entries addlw LOW(TABLE) ;add the base of the table skpc incf PCLATH,F ;oops, crossing 256 byte boundary movwf PCL TABLE: dt "A" dt "B" dt "C" dt "D" and then in the main code: movlw ">" call send_ascii ;display a ">" movlw 0 ;get first character in the table call fetchchar call send_ascii ; display it movlw 1 ;get second character in the table call fetchchar call send_ascii movlw "<" call send_ascii I think it makes more sense to place the table offset calculation along with the table. Scott PS. look at the archives for even better ways to display strings. -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu