Hello,

I am working with a friend on a system wich is able to give the datation=20
of an event with a precision of ms. For that we use the PPS (pulse per=20
second) coming from a GPS receiver. we have write a small program to=20
test the timer2 ability to give a 1ms interupt.
We know that the precision of the PPS is better than 5=B5s (precision=20
check with the atomic clock of Paris Observatory). So when the PPS come,=20
we write 0 in the register of the timer2 (for information Timer2 has=20
been initialized with a prescaler of 16, a postcaler of 1, and 249 in=20
PR2, wich give us an interupt every 1ms for a quartz of 16 MhZ). Then we=20
wait for the next PPS, when it comes, we write on rs232 the number of=20
1ms interupt and the residual value in Timer2 register. We should get=20
something like 1000 and a very smal value, or 999 and a value near 249.=20
But we get a value of 1000 and 34 for my system, and 1000 plus 42 for my=20
friend. We don't know why! The error for me is about 34*4=3D136=B5s, whic=
h=20
is far away of the mean error of a quartz!! My friend has tried with=20
other quartz, and it's strange because the value are allways 1000 plus=20
something quite high. If it was a problem of the quartz, we will have=20
values under and over 16MHz, but we allways have a value over 16MHz!?!
I think we make a mistake in the use of the Timer2, but I don't know wher=
e.
I have also make a test with a prescaler of 1, a postscaler of 16; and I=20
get 1000 plus 34, the same thing than previous test, it's impossible I=20
should have got something like 1000 plus (34*16) modulo 250 because the=20
register of timer2 run 16 times faster!

I have also join the small program that my friend has written to perform=20
these tests, it's in C but very simple:

/*      Test Timer_2 et PPS
        -------------------
prescaler :16
postscaler =3D 1
PR2 =3D 250 - 1

an interrupt every 1 mSec.

*/
#include <16F876A.h>
#device *=3D16

#use delay(clock=3D16000000)
#define CRYSTAL_FREQ    16000000

#fuses HS,NOWDT,PUT,BROWNOUT,NOWRT,NOLVP

#use rs232(baud=3D9600,parity=3DN,xmit=3DPIN_C6,rcv=3DPIN_C7,stream=3DPC,=
errors)

#use fast_io ( A )
#use fast_io ( B )
#use fast_io ( C )
#priority  EXT , TIMER2     //TIMER1

static long lmSecCount, lmSecCountOld, Timer2count;
static char Pulse, x;

/****  Interrupts   ******************************************/

#INT_TIMER2
   void Timer2Interrupt ( void )
      {
      lmSecCount ++;
      }

#int_ext
      //  PPS sur B0
    void intRB0 ()
      {
      Timer2count =3D get_timer2();
      set_timer2(0);
        lmSecCountOld =3D lmSecCount;
        lmSecCount=3D0;
      Pulse ++;
      }

/*********   Main  *************************************************/

    void main ( void )
       {
      /* INITIALIZE */
        port_b_pullups(TRUE);
      setup_adc_ports ( NO_ANALOGS );

      set_tris_b(0xff);
      set_tris_a(0x20);
        set_tris_c(0b10011010);

      setup_timer_2(T2_DIV_BY_16,249,1);
      set_timer2(0);
      fprintf(PC,"Event ");

      /* INTERRUPTS */
        enable_interrupts ( INT_EXT );     // PPS interrupt
      enable_interrupts (INT_TIMER2);    // enable timer2 interrupts
      enable_interrupts ( GLOBAL );       // enable all interrupts


      output_high(PIN_A2);
        Pulse =3D 0 ;
        while (Pulse < 3)
            {
            }
      fprintf(PC,"ready :  ");
      Pulse =3D 0 ;
        output_low(PIN_A2);    // a LED is switch on to show that PPS is=20
Ok now


       /* MAIN LOOP */

      while ( TRUE )
          {
         if (Pulse =3D=3D 1)
               {
               fprintf(PC,"%04lu %04lu ",lmSecCountOld, Timer2count);
               Pulse =3D 0;
               }

         }
      }



Thank you to have write me, and also for your help if you have an idea.

Best Regards

Michel Meunier

PS: sorry for my bad english!!

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist