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How to calculate PWM freq

 
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Ringo42



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How to calculate PWM freq
PostPosted: Fri Aug 27, 2010 1:18 pm     Reply with quote

I know this has been discussed before, but I cannot find the thread. Can someone explain an easy way to calculate the values needed to set a specific freq on the PWM pins? I know it is setup_timer_2, but reading the CCS manual and the pic datasheet does not help alot.
I'm using a 452 or a 4520 at 20Mhz. I want to set up freqs from 1khz to 4khz. What I need is a simple explanation of what each of the three terms really mean. Prescale, period, and postscale.

Thanks
Ringo
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Ringo Davis
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PostPosted: Fri Aug 27, 2010 1:27 pm     Reply with quote

PWM frequency formula:
http://www.ccsinfo.com/forum/viewtopic.php?t=17993

Duty cycle formula:
http://www.ccsinfo.com/forum/viewtopic.php?t=42417&start=1

PWM postscaler explanation:
http://www.ccsinfo.com/forum/viewtopic.php?t=29786&start=1
Sample code for PWM postscaler:
http://www.ccsinfo.com/forum/viewtopic.php?t=41473&start=3
Ringo42



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PostPosted: Fri Aug 27, 2010 1:45 pm     Reply with quote

thanks guys, that is what I needed.

I made a little 1 line spreadsheet to help me out. Is there any way to post it here?
Ringo
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Ringo Davis
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PostPosted: Fri Aug 27, 2010 1:58 pm     Reply with quote

Someone else has posted a spreadsheet in the Code Library:
http://www.ccsinfo.com/forum/viewtopic.php?t=42808

Here is a PWM calculator program:
http://www.ccsinfo.com/forum/viewtopic.php?t=40450

I haven't tried them.


Microchip AN539 also has some useful information:
Quote:
Frequency and Resolution Options for PWM Outputs

http://ww1.microchip.com/downloads/en/AppNotes/00539c.pdf
Ringo42



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PostPosted: Sat Aug 28, 2010 1:02 pm     Reply with quote

Can someone tell me what is wrong here? I get 1.2khz out, but it never changes to 4 khz. This is too simple to not work. What am I missing? Or can you not change the freq once it is set?

Ringo
Code:

#include <18f452.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)

void main()
{
    setup_ccp1(CCP_PWM);
    setup_ccp2(CCP_PWM);
    enable_interrupts(INT_TIMER2); // Timer 2 Overflow
    enable_interrupts(global);     // Enable IRQs
    while(1)
    {
        setup_timer_2(T2_DIV_BY_16,255,1);//1.2khz //
        set_pwm1_duty (498L);
        set_pwm2_duty (498L);
        delay_ms(1000);
        setup_timer_2(T2_DIV_BY_16,77,1);// 4 khz
        set_pwm1_duty (498L);
        set_pwm2_duty (498L);
        delay_ms(1000);
       
    }

}

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Ringo Davis
Ringo42



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PostPosted: Sat Aug 28, 2010 1:21 pm     Reply with quote

I guess it is something with my duty cycle. If I do this
Code:

void main()
{
    int x=0;
    setup_ccp1(CCP_PWM);
    setup_ccp2(CCP_PWM);
    enable_interrupts(INT_TIMER2); // Timer 2 Overflow
    enable_interrupts(global);     // Enable IRQs
    while(1)
    {
       for(x=255;x>70;x--)
       {
            setup_timer_2(T2_DIV_BY_16,x,1);//1.2khz //
            set_pwm1_duty ((128));
            set_pwm2_duty ((128));
            delay_ms(100);
       }
    }
}

Then it starts out at 50% duty cycle, but they duty cycle goes up as the freq goes up until it is just high all the time. What is the calculation for 50% duty cycle at varying freqs?

Ringo
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Ringo Davis
Ringo42



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PostPosted: Sat Aug 28, 2010 1:22 pm     Reply with quote

nevermind, got it, x/2. Makes perfect sense.
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Ringo Davis
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