A 24h timer using the zero-error algorithm on Timer0

[Jacques Nilo]

; This program display a 24h timer on a 2x16 LCD.
; It implements the zero-error 1 sec algorithm from Roman Black using Timer0
; See: http://www.romanblack.com/one_sec.htm
;
; This GCBasic version is derived from:
; http://www.sfcompiler.co.uk/wiki/pmwiki.php?n=SwordfishUser.SoftRTC
;
; JN May 2014
;
;Chip Settings
;#chip 16F1508,8       'Does not work with 16F1508. OSCCON redefined in init sub 
#chip 16F690,8         'Used with Real PIC simulator

;Defines (Constants)
#define LCD_IO 4
#define LCD_RS PORTB.6
#define LCD_NO_RW
#define LCD_Enable PORTB.5
#define LCD_DB4 PORTC.0
#define LCD_DB5 PORTC.1
#define LCD_DB6 PORTC.2
#define LCD_DB7 PORTC.3

On Interrupt Timer0Overflow Call IncCounter

Dim OneSecond,Cyc_Counter As Long                            ') These are the
Dim T0Load as byte alias Cyc_Counter                         ') 3 key
Dim Countdown as word Alias Cyc_Counter_U,Cyc_Counter_H      ') lines !
Dim hrs,mins,secs as byte
Dim update As Bit

OneSecond = 2000000-2           ' 1sec = 8MHZ/4 - TMR0 startup adjustment
;
; The following routine is called after each Timer0 overflow
;
sub IncCounter
   Countdown --
   If Countdown = 0 Then
      Cyc_Counter = OneSecond   ' reload timer
      TMR0 = TMR0 + T0Load      ' include additional TMR0 counts
      If STATUS.0 = 0 Then
         Countdown ++           'if no carry increment "countdown" counter once
      End If
      set update on             ' flag to update clock output
   End If
End Sub
;
; Initialization
;
Sub Initialize()
CLS
secs = 0
mins = 0
hrs = 0
Cyc_Counter = OneSecond
set update off
OPTION_REG = 0b00001000       'PSA on so no prescaler on Timer0
INTCON = 0b10100000           'GIE on, T0IE on (turn interrupts on)
; Following statement only needed for 16F1508
;OSCCON = 0b01110000           'Insure running at 8Mhz with internal osc on 1508
TMR0 = 0                      'Timer0 set to 0
End Sub
;
; Display a 24H timer on a 2x16 LCD display. Format is HHhMMmSSs
;
Sub Clock24()
    secs ++
    If secs = 60 Then            'Check each tally for rollover
       secs = 0
       mins ++
       If mins = 60 Then
          mins = 0
          hrs ++
          If hrs = 24 Then
             hrs = 0
          End If
       End If
    End If
    locate 1,0
    Print pad0(hrs)+"h"
    Locate 1,3
    Print pad0(mins)+"m"
    Locate 1,6
    Print pad0(secs)+"s"
    set update off
End Sub
;
; Does what it is named after :-)
;
function pad0(in var) as string
  if var < 10 then
    pad0="0"+str(var)
  else
    pad0=str(var)
  end if
end Function
;
; Main Program
;
Initialize
Do
  if update then
    Clock24
  end if
Loop

[Anonymous]

Thanks! I'm looking forward to playing with this, timekeeping being one of my hobbies.

[Jacques Nilo]

I am also discovering the fascinating world of timers :-)
I changed the code of my initial post to replace the 16F1508 by a more traditional 16F690 in order to avoid the OSCCON compile problem with this chip. I then compiled the stuff, imported the Hex file into real pic simulator, initialized a virtual LCD screen with the code ports setup (RS=PORTB,PIN6 / E=PORTB,PIN5 / D4=PORTC,PIN0 ...) and run. Et voila ! Here is the result and I was then able to check my clock against the one of my PC...

[Chuck Hellebuyck]

Timers can be really interesting to work with.
Recognize though that when you run it in the simulator it's using the PC clock as the time base so they should match up. When you run it on an actual PIC it will be less accurate over time due to the crystal accuracy.