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Highest PWM is only 2.5 Mhz

 
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pic-e



Joined: 29 May 2008
Posts: 1

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Highest PWM is only 2.5 Mhz
PostPosted: Thu May 29, 2008 7:12 am     Reply with quote
Hello,
I have a 16F877 running at 20Mhz. Using the formula below, I would think that I should be able to get out a freq of 5 Mhz but it is only 2.5.

Code:
                      Crystal Frequency
PWM Freq  = -----------------------------------------
                  (PR2 + 1) * (Timer2 Prescaler) * 4
 

My code:
Code:

#include <16F877.H>
#fuses HS, NOWDT, NOPROTECT,  PUT, NOLVP
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

main()
{ 
output_low(PIN_C2);  // Set CCP1 output low setup_ccp1 (CCP_PWM);             // Configure CCP1 as a PWM
setup_timer_2(T2_DIV_BY_1, 1, 1);       

set_pwm1_duty(1);   //  50% duty cycle
while(1);                  // Prevent PIC from going to sleep
}
RLScott



Joined: 10 Jul 2007
Posts: 465

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Re: Highest PWM is only 2.5 Mhz
PostPosted: Thu May 29, 2008 8:09 am     Reply with quote
The period of Timer 2 is (PR2+1) * Prescaler * 4 * Tosc. In your case Tosc = 50 nsec. With PR2=1, this works out to 2*1*4*50 nsec = 400 nsec. Therefore the frequency is 1/400 nsec = 2.5 MHz.

Robert Scott
Real-Time Specialties
Ttelmah
Guest
PostPosted: Thu May 29, 2008 8:55 am     Reply with quote
Code:

   setup_timer_2(T2_DIV_BY_1, 0, 1);
   set_pwm1_duty(2L); // 50% duty cycle


Best Wishes
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