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PWM Newbie

 
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sokrates



Joined: 14 Mar 2007
Posts: 12

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PWM Newbie
PostPosted: Fri Mar 16, 2007 7:11 pm     Reply with quote
Hello,

I am trying to start learning PWM techniques using 18F452.

The first simple task is to generate a fixed duty cycle- square wave using CCP1 pin of my PIC.

I simply use the following code:
( I am sorry if my question is very simple...)
Code:
#include <18F452.h>
#device adc=8

#FUSES NOWDT                    //No Watch Dog Timer
#FUSES HS                       //High speed Osc (> 4mhz)
#FUSES NOPROTECT                //Code not protected from reading
#FUSES BROWNOUT                 //Reset when brownout detected
#FUSES PUT                      //Power Up Timer
#FUSES NOLVP                    //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O


#use delay(clock=10000000)



void main()
{
  while(1)
  { set_tris_c(0);
   setup_ccp1(CCP_PWM);
   set_PWM1_DUTY(10);
   setup_timer_2(T2_DIV_BY_1,0,1);
  }
 
 }


There are several unclear points for me here.
1. This code simply produces a DC offset and there is no variation in this voltage.

2. I just see a square wave-form in the beginning and it converges to a DC offset. If I will "manually" configure PWM, what's the point of all these functions?

Please tell me my obvious mistakes.

Thanks in advance

sokrates
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Mar 16, 2007 8:19 pm     Reply with quote
See the PWM demo program in this post:
http://www.ccsinfo.com/forum/viewtopic.php?t=17729&start=1
sokrates



Joined: 14 Mar 2007
Posts: 12

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checked the data-sheet
PostPosted: Fri Mar 16, 2007 8:44 pm     Reply with quote
Thanks PCM Programmer Team.

I checked the Data sheet and completely understood how this works.

I didn't set the PR register in my initial code that's why I could never clear CCP output pin resulting in an always high output.

However, I now have another problem.

I want to use PWM output to control a servo motor and the minimum frequency ( about ~ 600 Hz ) in my case is just too high.

I think I can use "post scaling" but data sheet does not have a comprehensive description about it.

Can you guys help me ?

Thanks!
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Mar 16, 2007 8:59 pm     Reply with quote
Quote:
Thanks PCM Programmer Team.

I'm not a team. I'm one user of the CCS PCM compiler.
I don't work for CCS.

See this thread about the PWM postscaler. It doesn't affect PWM frequency.
http://www.ccsinfo.com/forum/viewtopic.php?t=29846&highlight=postscaler+pwm


See my post in this thread. It has a formula to calculate the PWM
frequency. You should be able to do a frequency close to 600 Hz.
http://www.ccsinfo.com/forum/viewtopic.php?t=17993
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