Suggestion on table lookup for LCD display

Discussion in 'Embedded Systems and Microcontrollers' started by dmarciano, May 4, 2012.

  1. dmarciano

    Thread Starter Member

    Oct 10, 2007
    I'm going to be using a Hantronix LCD in a project and this requires sending each column of pixels one at a time. I was planning on using a lookup table to send each letter or word. However this is my first time using lookup tables so I'm hoping someone can provide a suggestion. This code is simplified and was designed to help me learn lookups. Basically this code should read the hex code for the letter 'H'. After loading each byte the code jumps to a section which will actually send it to the LCD and then continue the loop. Once it has read all the data it should loop back to 'Start' (at least in this test version)

    Code ( (Unknown Language)):
    2. Start   movwf   tt,0
    3.         movlw   low(H_CHAR)
    4.         addwf   tt,0,0
    5.         movwf   TBLPTRL,0
    6.         movlw   high(H_CHAR)
    7.         movwf   TBLPTRU,0
    8. lpl     tblrd   *+
    9.         movlw   .153
    10.         CPFSLT  TABLAT
    11.         GOTO    Start
    12.         GOTO    Send
    14. Send    movf    TABLAT,0,0
    15.         movwf   PORTA
    16.         GOTO    lpl
    18. H_CHAR  db  0x00,0xfe
    19.         db  0x10,0x10
    20.         db  0xfe,0x99
    My problem is figuring out when it has read all the data. I was think of using 0x99 but realized I can't because if I wanted to enable all the pixels in a single column that would be 0xFF which is the highest, and if I wanted a column with no pixels on I would need to send 0x00, so any number in between these two values is possible.

    What is a good way so determine when I have read all the data in the 'H_CHAR' table (in this example)? The 0x99 is not part of the actual data being sent to the LCD screen. After reading the last 0xFE and jumping to 'Send', when it returns to the 'lpl' loop the code should somehow be able to figure out/know that all the data was read.

    This table is only 5 bytes, but some may be more and some may be less so it would help to have a single way of detecting this. Any help would be greatly appreciated. The application note for the LCD I'll be using can be found here and on the last few pages it give a sample code, albeit the code is in 8051 assembly which I'm not familiar with.
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I did a display similar to this a few years ago. Character bit patterns were handled as simple uniform sized bitmaps, so all you need do is know the character size and the character code you want and you can compute an offset into an array.

    I did spend much time on this and while I had a nice library of functions, I've abandoned that effort after finding simple black & white displays are not substantially cheaper then color displays.

    I now just use the color ones and the excellent, free graphics object library from Microchip.
  3. dmarciano

    Thread Starter Member

    Oct 10, 2007
    On Microchips website the graphics library is listed as only working with PIC24/dsPIC and PIC32, while I'm using the PIC18F46K22. I already have this chip and the Hantronix screen so I'd rather stick with them since they are sufficient for what I'm doing and I already have them in hand.

    I'm not sure exactly what you mean by using sized bitmaps along with any array to display character on the Hantronix screen. This is the first time I'm using an LCD screen so maybe I'm missing something. Do you have an resources that you could point me to that will explain how to use your way, or the way I was trying to do it, for the type of application I'm working on.

    Every time I try to Google table lookups they are about changing programming memory or the user has a special code at the end of the table they can check for; which I can't do for the reasons already stated. Is there a function for returning the size or element count of a lookup table?
  4. Markd77

    Senior Member

    Sep 7, 2009
    What I've done in the past is make the first byte of each array the length.
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    I make fonts from bitmaps of the same size placed into an array and padded out do each character starts on an even byte field. I used this structure for fonts:
    Code ( (Unknown Language)):
    1. typedef struct
    2. {
    3.     rom char FirstFont;            // first ASCII symbol defined
    4.     rom char LastFont;            // last ASCII symbol defined
    5.     rom char DefaultFont;        // If nothing available...
    6.     rom char FontWidth;         // font width in pixels
    7.     rom char FontHeight;        // font height in pixels
    8.     rom char BytesPerCharacter;    // bytecount for a single character
    9.     rom char *Bitmap;             // first byte of ROM bitmap
    10. } font;
    Then a font file would look like so:
    Code ( (Unknown Language)):
    1. rom char Font5x7bits[] = {
    2.     0x00, 0x00, 0x00, 0x00, 0x00, 0x00,        // Space    32
    3.     0x00, 0x00, 0x4F, 0x00, 0x00, 0x00,        // !        33
    4.     0x00, 0x07, 0x00, 0x07, 0x00, 0x00,        // "        34
    5. ...
    6. ...
    7.     0x00, 0x01, 0x02, 0x04, 0x00, 0x00,        // `        96
    8.     0x7F, 0x41, 0x41, 0x41, 0x7F, 0x00        // (box)    97
    9. };
    11. rom font _Font5x7 =
    12. {
    13.       32,        // first ASCII symbol defined
    14.       97,        // last ASCII symbol defined
    15.       97,        // If nothing available...
    16.       6,         // font width in pixels
    17.       8,         // font height in pixels
    18.       6,        // 6 bytes in each character definition
    19.       Font5x7bits
    20. };
    Obviously there are a bunch of routines behind this, and the bad news is since this is an abandoned work whatever documentation existed was never completed and pretty much lost. I think I did a brief explanation of the GLCD functions but that's gone. I also did a VB applet to take B&W bitmaps and convert to C style byte arrays, that's also gone.

    I do have one of the last projects I did and I've included it in case you wish to reverse engineer some of it.
      File size:
      308.5 KB
  6. nigelwright7557

    AAC Fanatic!

    May 10, 2008
    I have always used the same number of bytes for each character.