Re: [Gumstix-users] digital servo drive

[Dave H. <dhy...@gm...>]

Hi Rodrigo,

> reading more in depth how PWM is generated at the mega128, guess it wont
> work as is for a digital servo.
> whereas a standard servo works like this and from what i undersantd, the
> mega128 generates PWM as:
> _____                     _____
> |       |                     |       |
> |       |                     |       |
> |       |____________|       |____________
> <-------------------------> <-------------------------->
>      cycle 20ms                  cycle 20ms

That's just one of many methods that PWM can ge generated,

> A digital servo signal expect a PWM like this (the widht of pulse also
> varies like standard)
> __  __  __  __  __  __  __  __  __  __  __  __
> |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
> |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
> |  |_|  |_|  |_ |  |_|  |_|  |_|  |_ |  |_|  |_|  |_|  |_ |  |
> <---------------------------><------------------------------>
>      cycle 20ms                  cycle 20ms
> So to sum up a digital servo expect a more spreaded signal in the same cycle
> of 20 ms
> As of what i read, it expects 6 pules in one cycle so in one second a
> standard servo receives 50 pulses and a digital one 300.
> So to set the PWM generator i should generate a shorter pulse 6 times like
> in a loop what do you think?

Change the 40000 to 40000/6 = 6667, and you'll get 6 times as many
pulses in the same time period.

> On the other hand, in the script, you are using fast PWM with compare output
> mode right?
> so on compare match OCnA/OCnB/OCnC will clear and set at TOP, this is set on
> TCCRnA
> and then you set TCCRnB fot Fast PWM with ICRn for saving TOP and prescaler
> 8
> I guess i understand now the script, but setting TOP at 40000 means a PWM
> cycle of 40 milisecs? wouldnt it be about 20 milisecs the needed? or am i
> doing something wrong?

The 40000 means 40000 ticks. With a 16 MHz clock and a divide by 8
prescalar, this means 2000000 ticks/sec, which means each tick is 1/2
usec. 40000 * 1/2 = 20000 usec = 20 millisec.

-- 
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/