Dear All


can any one explain to me
How to build a clock using Microcontroller timer??


Do you use external crystal oscillator or you use the Microcontroller
timer to do the time keeping??


Thanks for your help


Sam

Sam,
Below are some timing routines that I reuse frequently in my PIC coding.   
 (Attention Code Police and Piclister's, please check the readability and   
math).  Building a clock is simply a matter of gluing these type of   
routines together with a few counters to keep track of minutes, hours,   
etc, and some kind of display mechanism.

Another method of doing the timing (with a PIC device that supports   
interrupts) would be to set up TMR0 and the prescaler to fire on known   
intervals and increment your counters in the interrupt service routine.

An external crystal or ceramic oscillator is essential, whether or not   
the PIC you are using has an internal RC oscillator.  (The 16C84 has NO   
internal clock, while the lowly 12C508's and 9's have an internal RC, go   
figure).  The problem with the internal RC (or any RC network unless you   
use very expensive components) is it's limited accuracy (+-10%) and the   
fact that the accuracy varies depending on temperature.  The accuracy   
variance may also vary between 2 identical PIC devices making it   
difficult or impossible to calibrate your clock if you are producing   
multiple units.

Hope this gives you a start...
Adam
;*************************************************************************  

; TIMER ROUTINES
;*************************************************************************  


        ;this routine implements a 1/2 second timing loop
 ;Delay calculation is as follows:
 ; .500420 - by calling the 25ms loop 20 times.
 ; .000100 - delay of this loops loop housekeeping
 ; .500520 - subtotal
 ; .000005 - delay for this loops setup
 ; .500525 - total delay for this loop
delay500
 MOVLW 20
 MOVWF longcount
 CALL delay25
        DECFSZ  longcount,1
        GOTO    $ - 2
        RETLW   0

        ;this routine implements a 25ms timing loop.
 ;Delay calculation is as follows:
 ; .023836 - by calling the 1/10ms loop 236 times.
 ; .001180 - delay of this loops loop housekeeping
 ; .025016 - subtotal
 ; .000005 - delay for this loops setup
 ; .025021 - total delay for this loop
delay25
        MOVLW 236
 MOVWF count
 CALL delay1_10
        DECFSZ  count,1
        GOTO    $ - 2
        RETLW   0

 ;this routine implements a 1/10 millisecond timing loop
 ;Delay calculation is as follows:
 ; .000096  - 4 cycle loop 24 times.
 ; .000005  - delay for this loops setup
 ; .000101  - total delay for this loop
delay1_10
 movlw   d'24'
 movwf   qmcount
 nop                     ;needed to make the loop a nice even 4 cycles
 decfsz  qmcount,1
 goto    $ - 2
 retlw   0