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'[PIC:] Tri-stating outputs'
2004\07\14@223359 by Bruce Douglas

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There is a small pdf on Microchip's site called "Better PWM Linearity"
(MP077) where the author recomends tri-stating a pwm output when you
have some slow processing to do which will significantly mess up the pwm
cycles.

I am guessing that I have to switch my pwm outputs to inputs to do this.
 Is this correct?

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2004\07\15@025619 by hael Rigby-Jones

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>-----Original Message-----
>From: Bruce Douglas [.....bdouglasKILLspamspam@spam@UOL.COM.BR]
>Sent: 15 July 2004 03:33
>To: PICLISTspamKILLspamMITVMA.MIT.EDU
>Subject: [PIC:] Tri-stating outputs
>
>
>There is a small pdf on Microchip's site called "Better PWM Linearity"
>(MP077) where the author recomends tri-stating a pwm output
>when you have some slow processing to do which will
>significantly mess up the pwm cycles.
>
>I am guessing that I have to switch my pwm outputs to inputs
>to do this.
>  Is this correct?


Correct, but this will only help if you have a low pass filter ( RC Circuit)
on the PWM output to get an average DC level.  Tri-stating the output will
ensure the voltage across the capacitor does not change significantly whilst
you perform any processing that messes up the PWM period.

Note that this is only required if you are "bit bashing" PWM in your main
loop.  If you are using hardware PWM (CCP module) or software PWM via timer
interrupts then main loop CPU utilisation should have no effect.

Regards

Mike

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2004\07\15@034241 by Matthew Brush

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Bruce Douglas wrote:

> There is a small pdf on Microchip's site called "Better PWM Linearity"
> (MP077) where the author recomends tri-stating a pwm output when you
> have some slow processing to do which will significantly mess up the pwm
> cycles.
>
> I am guessing that I have to switch my pwm outputs to inputs to do this.
>  Is this correct?

I'm no expert, but from what I've learned, the three states are high,
low, and float.

Maybe s/he means to let an external resistor pull the PWM output high or
low as opposed to having the PIC do it?

Good luck, cheers

MJ Brush

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