Neil wrote:

> On 5/13/2012 11:13 PM, Harold Hallikainen wrote:
> > > From your description, it sounds like you're using an adc to measure =
the
> > output voltage and then setting the output bit high or low as appropria=
te.
> > This is similar to delta modulation. Is this what you're doing?
> >
> > I think the random number approach did not use an ADC. Instead, the
> > desired output is compared to random numbers and the pin set high if th=
e
> > desired output is above the random number and low if it's below. If the
> > random number were not random, but were, instead, a steadily increasing
> > number, you'd get standard PWM. But, you would have to increment the
> > number at fixed intervals to get the required duty cycle. As I read thi=
s
> > random number method, you can call run it at irregular intervals and th=
e
> > output will still be correct due to the use of the random numbers.
> >
> > It's a really interesting idea.
> >
> > Harold
>
> Yes, that's pretty much what I'm doing... I think I'm missing part of a
> discussion somewhere where that random number technique was explained as
> the technique I used, and after thinking and thinking I could not figure
> out the purpose for random numbers.  I'll have to find that other
> discussion with the technique I used.
>
> Curious though -- what's being used to *compare* the random number to
> the desired output?

I wrote the original random number ATOD post a few years ago. I used the
random number ATOD trick to create a PWM on a few projects including
the tone generator for a DTMF dialler..

To understand the random ATOD start by reviewing how a normal timer based
PWM works. The output voltage is equal to the time on divided by time off
multiplied by voltage swing between 1 and 0 logical levels plus logic 0 off=
set.

A random number ATOD also creates a an output ratio of 1's and 0's. The
ratio of output 1's to the total bits The output voltage is equal to the ra=
tio
of 1's to the total 1's and 0's multiplied by the voltage swing  between 1 =
and
0 logical levels plus logic 0 offset.

They will produce the same output voltage.

To create a Random number PWM.

1) Decide the range of the random number RandRange typically 2^8 or 2^16 .

2) The output voltage when output is logical 1 is Vout1 and output voltage =
when output is logical 0 is Vout0;

3) Desired Output voltage is Vout somewhere between Vout0 and Vout1

4) Calculate a random threshold number. The ratio of RandomThreshold  / Ran=
domRange is equal to the desired output Vout / (Vout1 - Vout0)

5)  RandomThreshold =3D (Vout - Vout0) / (Vout1 -Vout0) * RandRange .

6) Generate a random number as often as possible and compare it to the Rand=
omThreshold. If the RandomThreshold is higher or equal to the random number=
 output a 1 else output a 0

Thats it.

I typically used a 8 bit random number for fast moving data like the DTMF a=
nd 16 bit random numbers
for slow moving data. The  resolution is very similar to a timed PWM.

Advantages of random PWM, the noise is easier to filter because it has high=
er frequency components than the timed PWM.

Disadvantages to the random PWM. It is computationally more intensive than =
a timed PWM.


Regards,


Walter..
--
Walter Banks
Byte Craft Limited
http://www.bytecraft.com





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