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itsbasix
Joined: 23 May 2012
Posts: 8
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| PWM duty cycle with PIC18F4550 |
 Posted: Mon Jul 08, 2013 9:51 am |
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Hi guys,
i know that this topic has been already discussed in many post, but i still have some doubts about it to be cleared.
I'm using a PIC18F4550 in USB CDC mode, with a 20MHz crystal and PLL to get a 48MHz frequency.
In order to get a 12KHz PWM freq with CCP module I set Timer2 like this:
| Code: | | setup_timer_2(T2_DIV_BY_4,249,1); |
I know that T_high in PWM is calculated as:
| Code: | | T_high=value*(1/clock)*t2div |
( where value is chosen when calling set_pwmX_duty(value) function ).
Then, if i need to set the duty cycle generated from CCP module i calculate value as it follows and call set_pwmX_duty(value):
| Code: | value= T_high*clock/t2div;
set_pwmX_duty(value); |
With my current configuration, for a 100% duty cycle, i should set:
| Code: |
T_high=(1/12000)=8.333333*10^(-5)
1 * 48*10^(6)
value = --------------------- = 1000
12*10^(3) * 4
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what i don't understand is this: wasn't 1023 supposed to be the value to get 100% duty in any case? Am i making any mistake?
Thank you and sorry for my bad english. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jul 08, 2013 11:10 am |
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| Quote: | | wasn't 1023 supposed to be the value to get 100% duty |
No. The PIC data sheet doesn't say anywhere that the duty cycle is
based on some power-of-2 value. It's based on the Timer2 reload value. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19537
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| Posted: Mon Jul 08, 2013 11:34 am |
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The maximum value for the duty cycle, is always ((PR2+1)*4-1).
So with your timer setup, and PR2=249, 999.
The point about 1023, is with the maximum PR2 value (255), this gives 1023.
So there is no magic about 1023, except it is the maximum duty cycle value, _when the maximum PR2 value is selected_....
Best Wishes |
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itsbasix
Joined: 23 May 2012
Posts: 8
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| Posted: Tue Jul 09, 2013 1:03 am |
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thank you for your answers!
Have a nice day  |
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