Virtual 20x4 display in ram on a 12F683

[Mike, K8LH]

I did something like this but it was for a 12F1822 with a 2x16 LCD display using the 4-bit interface and 250 microsecond update intervals.

Using a faster update interval and modifying the buffer directly does allow you to do some neat things. For example, exclusive-or'ing the colon character positions in the buffer with ':'^' ' every 500 milliseconds will produce the flashing colon effect seen in the video below.



Here's the interrupt routine from the demo (C code);

  /******************************************************************
   *                                                                *
   ******************************************************************/

   void interrupt()             // 250 usec (500 cycle) interrupts
   { static char line = 0xC0;   //
     static char bndx = 0;      //
     pir1.TMR2IF = 0;           // clear TMR2 interrupt flag

    /*                                                              *
     *  update one LCD character from the display array buffer (34  *
     *  interrupts or 8.5 ms to completely refresh the display).    *
     *                                                              */
     if(line & 0x80)            // if "new line"
       PutCMD(line ^= 0x40);    // set DDRAM address (0x80 or 0xC0)
     else                       // not "new line" so
       PutDAT(lcd[bndx++]);     // refresh display, bump index
     bndx &= 31;                // pseudo %32, 0..31 inclusive
     if((bndx & 15) == 0)       // if new line (0 or 16)
       line ^= 0x80;            // toggle b7 pseudo "new line" flag
   }