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Author Topic: Software PWM  (Read 1836 times)
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king
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« on: July 13, 2009, 11:09:14 11:09 »

Hi!
  I have written the code for software PWM whose PWM frequency is 20KHz

Method:
1) Generate HPWM1 and HPWM2 of 20KHz frequency using HPWM command
2) wait for TMR2 to PR2 match interrupt Flag
3) when this flag=1 then high PWM3 port pin and start timer1 whose max delay is 50usec (20KHz)
   but for 50% Duty Cycle I have reduce Timer1 delay to 25Usec by setting its TMR1H and TMR1L variables
    and jump to Step '1'

When Timer1 OV interrupt generated then i have low PWM3 Port Pin so its frequency is equal to 20Khz but i have received 0.146V on PWM3 port pin instead of 2.5V and it is not working correctly , i have also attached the code if anybody find the mistake then it will be highly appreciated
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Wizpic
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« Reply #1 on: July 13, 2009, 07:24:28 19:24 »

I've had a quick look at your code and can see a few error's, I shall a more indpeth look later and upload some code that shoudl do it

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TomJackson69
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« Reply #2 on: July 14, 2009, 01:28:35 01:28 »

Hi king,

I don't have Basic Compiler install in my computer so I can not compile your program. But I see one could be mistake in your program.

In the data sheet it said:
"The prescaler counter is cleared on writes to the TMR1H or TMR1L registers".

It could be true because your duty cycle now is only 12.4uS instead of 49.6uS. Which mean the LED on time is 1/4 of what you have planned.

So in your TMR1_ISR, after you write new value into timer1, set bits for prescale too.

TMR1_ISR:
LOW PWM3
LOW led
timer1=65473   '(50% DUTY CYCLE of 50US=25usec)

T1CKPS0=1      ' set prescale bits for 1:4
T1CKPS1=0      ' set prescale bits

FTMR1=0
CONTEXT RESTORE



Also, in the data sheet said:
"For writes, it is recommended that the user simply stop
the timer and write the desired values. A write contention
may occur by writing to the timer registers, while
the register is incrementing. This may produce an
unpredictable value in the timer register". I doubt it would help???

In this line:
SYMBOL FTMR1 = PIr1.0       ' TIMER1 OVERFLOW FLAG
Is PIr ok with your compiler? I don't know if its case sensitive.

Tom
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