On Dec 18, 2013, at 11:49 PM, Lindy Mayfield wrote:

> it's still not clear, what duty cycle is.

> The frequency is the time between pulses.

Technically, the PERIOD is the time of one cycle of the waveform (same as t=
he "time between pulses.")
The FREQUENCY is the number of cycles you get in a particular time (one sec=
ond.)
They're intimate related; f =3D 1/p

> The duty cycle is the percentage of time the line is high.  So I guess th=
at a perfect square wave would have a duty cycle of 50%?

Yes.  Exactly.


> PWM is simply changing the duty cycle from 1 - 99%

0 to 100%, ideally.  But you have the right idea.  Your desired waveform (1=
0us every 1s) has a duty cycle of 0.001%


> Normally the frequency is 20 ms?

20ms is a typical period for Radio Control signals (ie to servos.)  I didn'=
t think that was what we were talking about.
There are various reasons why RC signals aren't quite "typical pwm."


> So when you say "high duty cycle" do you mean the time the line is high c=
ompared to low?  So like 75% would be high?

Yes.  The 555 in "astable" (oscillator) mode tends to produce waveforms wit=
h greater than 50% "high" time.
In monostable (pulse) mode it produces output "low" pulses when triggered. =
 Whether you set up a really extreme duty cycle oscillator (not too likely =
to work) or a double-555 configuration where a slow oscillator triggers a s=
hort pulse generator, the output waveform is going to have short "low" time=
s and long "high" times, which is opposite what you said you wanted.


> why do you need to invert a square wave?

You wouldn't.  If you did the double-555 setup, you'd only need to invert t=
he final output, not the (more square-shaped) wave between the two 555s.

BillW



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