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Nov 09, 2005 07:46 PM Please respond to
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To
AVR-Chat@yahoogroups.com cc Subject RE: [AVR-Chat] Phase and Frequency Correct PWM on a Mega 8




The simulator was my first suspicion but I programmed a chip (using a large prescalar) and got nothing. I'm sure it's something I'm doing wrong but darned if I can find what.

Martin Jay McKee

>>Also if you want to see how to use that technique to get 8 r/c pulse
outputs from two pins, look at:

 

>>http://www.barello.net/Papers/AVR%20RC%20output.pdf

 

I saw this a while ago and I was meaning to ask (never got around to it) how
smoothly it works.  It seems that the time between pulses would vary
slightly and so allow for the possibility of jitter.  Has that shown up at
all in use?

 

Martin Jay McKee

While the time between the pulses may move around a bit, this causes 
no problems to a servo.  It is the pulse width of the pulse that 
matters, not the period.  That said, with analog servos a faster update 
rate (period) makes them stronger, a slower one makes them weaker.  I've 
experimented with this and found that you can't make the update period 
too fast or the servo will jitter.  When going slower however what 
happens is that the servo will weaken until it does not respond any more.

DLC

Martin Jay McKee wrote:
>> >Also if you want to see how to use that technique to get 8 r/c pulse 
> outputs from two pins, look at:
> 
>  
> 
>> >http://www.barello.net/Papers/AVR%20RC%20output.pdf
> 
>  
> 
> I saw this a while ago and I was meaning to ask (never got around to it) 
> how smoothly it works.  It seems that the time between pulses would vary 
> slightly and so allow for the possibility of jitter.  Has that shown up 
> at all in use?
> 
>  
> 
> Martin Jay McKee
> 
> 
> 
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-- 
-------------------------------------------------
Dennis Clark          TTT Enterprises
www.techtoystoday.com
-------------------------------------------------

There is no jitter between pulses as the timer match condition finishes one
pulse and starts the next. The processor has up to the minimum pulse width
to service the interrupt and set up the next match.

 

The only jitter would be the start of the pulse train and that would be
whatever jitter you have in your system time.  If, for example, you had a
1khz clock you could call "StartRCout()" from the interrupt handler every 24
ticks and have very low jitter.


But, besides that, Servos are pretty insensitive to the pulse repetition
rate.  Anything between 20-60hz would be fine.

 

 

-----------

Larry Barello

www.barello.net

 

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From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Martin Jay McKee
Sent: Saturday, November 12, 2005 11:32 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Servo Control Without PWM, Was: Phase and Frequency
Correct PWM on a Mega 8

 

>>Also if you want to see how to use that technique to get 8 r/c pulse
outputs from two pins, look at:

 

>>http://www.barello.net/Papers/AVR%20RC%20output.pdf

 

I saw this a while ago and I was meaning to ask (never got around to it) how
smoothly it works.  It seems that the time between pulses would vary
slightly and so allow for the possibility of jitter.  Has that shown up at
all in use?

 

Martin Jay McKee

 

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