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tapasparua
Joined: 13 Feb 2012
Posts: 1
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| Need help in pwm generation using CCS C |
 Posted: Mon Feb 13, 2012 5:54 am |
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I am trying to generate 2 pwm using ccp1 & ccp2 in pic16f877a. I am using the following instructions.
| Code: |
setup_ccp1(CCP_PWM);
setup_timer_2(T2_DIV_BY_1, 255, 5);
set_pwm1_duty(31);
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Q1. I am getting pwm in both ccp1 & ccp2. in the 2nd instruction,i.e.,
setup_timer_2(T2_DIV_BY_1, 255, 5);
i.e., setup_timer_2(T2_DIV_BY_a, offset, postscaler);
T2_DIV is prescaler, 255 is offset, 5 is postscaler.
prescaler & offset have role in calculating period (i.e., pwm freq). But I found that postscaler has got no role. I have simulated this in Proteus. There if I change the value of postscaler in 1-16 (this range is given in ccs c manual), the freq of pwm did not change. Why?
Q2. Duty cycle is 10bit (found in datasheet). That means in the 3rd instruction i.e., set_pwm1_duty(x); x can be varied from 0 to 1023, where 1023 means almost 100% duty cycle, and x=(duty cycle in %) x 1023. But in some documents it is written x=0 to offset in instruction 2. Which one is right? While simulating in Proteus, I have seen x=(duty cycle in %) x 1023. This formula does not work. x=(duty cycle in %) x offset works. That means I can not have 10bit resolution if I use ccs c.
Can anybody please help me to solve this? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19538
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| Posted: Mon Feb 13, 2012 6:20 am |
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The postscaler has nothing whatsoever to do with the frequency generated. Hence does not affect the PWM. _READ THE CHIP DATA SHEET_.
It affects how often the timer generates an interrupt _only_.
The pwm on both outputs, is a Proteus bug.
Then, _search the forum, and read the manual_.
A Search on 'PWM 10bit', with 'search for all terms', will find the hundreds of threads explaining what you are seeing. Ore read the set_pwmx_duty section in the manual, and read the paragraph 'function'. Note what happens if an 8bit value is used with the function....
Best Wishes |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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