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Pwm frequency problem

 
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joseph



Joined: 14 Jun 2010
Posts: 16

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Pwm frequency problem
PostPosted: Wed Oct 31, 2012 10:50 am     Reply with quote
Hi,

I working on pwm system and I have a problem. I try to understand some things about the pwm frequency:
By computing, I should have 3.311 kHz but I see 6.5khz with a scope.
Where is the mistake?
Thanks in advance

Code:

#include <18F2520.h>

#FUSES NOWDT,INTRC_IO
#use delay(internal=4000000)

void init();
//--------------------------------------
void main (void)
{
      init();
      set_pwm1_duty(20);
      
      for(;;){}
}
//--------------------------------------
void init()
{
      setup_timer_2(T2_DIV_BY_1, 150, 2);   
      set_timer2(1);

      setup_ccp1(ccp_pwm);
}
//--------------------------------------
Mike Walne



Joined: 19 Feb 2004
Posts: 1785
Location: Boston Spa UK

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PostPosted: Wed Oct 31, 2012 11:55 am     Reply with quote
No. I get 6.623kHz.

The timer2 is clocked at oscillator/4 = 1MHz.

You're then telling timer2 to reset after 151 clocks that's 1^6/151 = 6623Hz.

Then you're asking for interrupts at 6623/2 = 3311Hz.

It's all in the CCS manual.

Mike

EDIT Don't forget the tolerance on the internal clock when measuring with 'scope etc!
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Wed Oct 31, 2012 9:37 pm     Reply with quote
Quote:
setup_timer_2(T2_DIV_BY_1, 150, 2);

The last parameter of 2 is only used with the #int_timer2 interrupt.
You don't have an interrupt routine, so it has no effect on your program.

Links on PWM:
http://www.ccsinfo.com/forum/viewtopic.php?t=45968&start=1
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