On Saturday 19 April 2003 09:24 am, James Williams wrote:
> How do you calculate the out pulse duration?  In my example, I have
> a 0.01uf cap and a 15Kohm resistor.  The book I am reading states
> that the pulse is 200us, however when I calculate the RC time
> constant at the first time interval, I only get 150us.  This should
> have caused the circuit to have a pulse of 150us.  Because the
> first time interval is V is at 67% charge leaving the voltage at
> 3.35 V.  Or did I miss something else here too?
>

Yes, the threshold of the second inverter determines how far the cap
needs to charge.

If your second inverter switched at V+/3 you could indeed go for a
period of RC. However, it doesn't, probably, if it's CMOS.

So the period will be shorter, since the cap only has to charge over
50% of Vcc, not 67%.

The formula for the period is a simple exponential, since the rate of
cap charging depends on the voltage across the resistor. Assuming a
5V supply,

Vc = 5 * e ^ (-t/RC)

solving for Vc = 2.5 (the threshold for CMOS):

ln(0.5) = -t / RC
t = - ln(0.5) * RC
t = 0.693 RC

So your RC time constant of 150usec will result in a 104 usec pulse.

http://www.educatorscorner.com/media/Exp31.pdf

--
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.