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Frequency meter calculation error

 
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jojos



Joined: 30 Apr 2007
Posts: 64

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Frequency meter calculation error
PostPosted: Thu Jan 24, 2008 6:26 am     Reply with quote
Hello.I am working on a frequency meter project.Processor 18F2420 8 Mhz internal oscillator As signal i am using a 50 Hz square wave (5Vp-p) that comes from a frequency generator.
On the oscilloscope i can measure 50 Hz to the T0CKI pin.

I use Timer0 for external edge counter (high to low) T0CON=0b00111000; . Inside interrupt of Timer1 i calculate the frequency.
Timer 1 interrupt occurs every 40 msec.
The problem that i have is i get a 51 to 52 Hz indication and not 50 Hz as i should.Also as i raise the frequency the error gets bigger

I used two methods to calculate frequency.I had the same error in both of them.
Here is the first (simple) method that i calculate frequency.Every 1 sec i stop Timer0 ,read the T0 counter value that equals the frequency i need :

Code:


#INT_TIMER1

void service1(void)
{
   char butt=0;
   int16 temp=0,calc_pulse=0;


   disable_interrupts(INT_TIMER1);
   //disable_interrupts(INT_TIMER2);
   
   
   
   sec_timer++;   

   

   if(sec_timer==25)            //25 *40 msec = 1sec
{
   sec_timer=0;

   TMR0ON=OFF;             //Stop T0 during calculation
   pulses=make16(TMR0H,TMR0L);

   
   sec_pulses=pulses;
   SET_TIMER0(0);            //Restart Timer0 and set to zero its counter.
   TMR0ON=ON;

}   


   Bin2Bcd(sec_pulses);

   


enable_interrupts(INT_TIMER1);
//enable_interrupts(INT_TIMER2);
TMR1IF=0;
TMR1H=0xD8;            //40 msec set interrupt
TMR1L=0xEF;
//TMR1H=0x9E;
//TMR1L=0x57;

   } 




Here is the second method that i calculate frequency.Now i dont sytop - restart Timer 0 i let it in count all the time and every time i calculate freq i save the current value of Timer 0 (pulses variable) as
previous pulses ( pr_pulses variable) :


Code:


#INT_TIMER1

void service1(void)
{
   char butt=0;
   int16 temp=0,calc_pulse=0;


   disable_interrupts(INT_TIMER1);
   
   
   
   sec_timer++;   



   if(sec_timer==25)
{
   sec_timer=0;
   
   pulses=make16(TMR0H,TMR0L);

      
   sechigh_pulses=0;
   sec_pulses=0;
      if(pulses>pr_secpulses)
      sec_pulses=pulses-pr_secpulses;   
      
      if(pulses<pr_secpulses)
      {
         sec_pulses=0xFFFF-pr_secpulses;
         sec_pulses=pulses+sec_pulses;
         sec_pulses++;
      }   
   
   
      pr_secpulses=pulses;
      if(sec_pulses>999)
      {
      sechigh_pulses=sec_pulses/1000;
      sec_pulses=sec_pulses%1000;
      
      }

   
}   


   Bin2Bcd(sec_pulses);

   


enable_interrupts(INT_TIMER1);
//enable_interrupts(INT_TIMER2);
TMR1IF=0;
TMR1H=0xD8;
TMR1L=0xEF;
//TMR1H=0x9E;
//TMR1L=0x57;

   }




P.S. I changed from internal 8 Mhz to an external 8 Mhz and i get 50 -51 Hz indication instead of the previous 51- 52 Hz , but still there is an accurasy problem cause this 1 Hz error in 50 Hz it gets 3-4 Hz error on 200 Hz input frequency and so on

Thank you for your time.
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Thu Jan 24, 2008 11:46 am     Reply with quote
Use the forum search page to search for:
Quote:
tachometer

There are many posts with sample code and a discussion of how to use
the CCP module in Capture mode to find the frequency of a signal.
http://www.ccsinfo.com/forum/search.php
RLScott



Joined: 10 Jul 2007
Posts: 465

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Re: Frequency meter calculation error
PostPosted: Thu Jan 24, 2008 1:34 pm     Reply with quote
jojos wrote:
...there is an accurasy problem cause this 1 Hz error in 50 Hz it gets 3-4 Hz error on 200 Hz input frequency and so on


This would tend to indicate that your 50 Hz and 200 Hz are not accurate. How did you check them? An o-scope timebase is not good enough for the small errors you are talking about. The other thing to check is the accuracy of the 40 msec. Timer 1 interrupts. You could toggle an I/O bit every time you enter the Timer 1 ISR and then see if you really are getting exactly 25 interrupts per second.

Robert Scott
Real-Time Specialties
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