REAL TIME CLOCK ROUTINE

                             by Don Retzlaff



       Your Bigboard computer is ready to keep real time if you

       purchased the clock option.  Thanks to Russell Smith who wrote

       the guts of this Real Time Clock and Display routine, we have an

       excellent asset for your Bigboard.  The principle features of

       this routine are:


            1. Keeps time in Years, Months, Days, Hours, Minutes,

               and Seconds.


            2. Displays Hours, Minutes, and Seconds in the upper

               right hand corner of the screen.


            3. Clock can be read from high level languages.


            4. Clock can be set from high level languages or with

               the following SETCLK routine.


       The Clock routine starts with a move-up routine (described in

       another issue of Micro C.) to put the routine in the desired

       location in upper memory.  After you get it running you may want

       to change the origin, to pack your high memory routines.


       Next the interrupt vector is setup to the clock routine.  The

       interrupts are enabled and control is returned to CP/M.  Each

       time the clock ticks (every second) the clock increments the
       
       seconds counter.  After 60 seconds the minutes are incremented

       and the seconds set to zero.  This process is repeated for Hours,

       Days, Months, and Years.  Our timer does not compensate for Leap

       Year, to conserve memory.  When each second is updated the time

       is displayed in the upper right corner of the screen.


       NOTE:


            I have renamed locations FF5FH and FF61H from the names

            shown in your PFM-80 manual.  This was done to put all

            time values in natural order, to save memory.


            You will also note that when the day is updated, I had to

            do some fancy footwork to keep up with the varying length

            of the months.


       To make the counter work it is necessary to jumper the timer plug

       JB2 (3-4 and 7-8).  This strapping is also correct for the Disk-AC

       time out function.


       Don't forget that after you assemble the program you have to load

       the HEX file with DDT, move it to 0100H, then save it with the

       SAVE program as a COM file.


       On my two Bigboards I notice that the clock keeps time very well

       on one (+- 2 seconds per day).  The other gains about 40 seconds

       every 24 hours.  This can be corrected by replacing C106 with a

       variable capacitor 20-75 pf.  Set the frequency with a frequency

       counter.  (I haven't tried this yet, but it should work - If you

       try it, let me know how bad I blew it.)