Joe,
     I have a 20MHz crystal. That is what I am thinking about the pot=20
but I could include a switch as a hold signal. With the switch on the=20
back of the pot you could just turn the pot until the switch activates.=20
I'm building a frequency counter that uses 74390s to accumulate the=20
count and cd4511s to display the count on 7 segment LEDs. The one second=20
high pulse opens the gate to send the input signal to the 74390s. One=20
second =3D HZ.
     The 74390s require at least a 20ns high pulse to clear the count.=20
Right now I am just holding the strobe line on the 4511s low and this=20
allows the count to be transferred from the 390s to the 4511s as the=20
count comes in. When the gate closes the counting stops. The control=20
gate is just a two input NAND gate. One input is connected to the input=20
circuitry and the other input is the one second gating signal from the PIC.
Thanks,
rich!

On 7/18/2014 2:19 AM, IVP wrote:
>> #1  Pulse has to be high for one second plus/minus =85. how
>> much precision ?
> 20MHz crystal will allow down to 200ns
>
>> #4  You can use a potentiometer into an ADC.  I guess you would
>> want to translate the incoming voltage into a logarithmic time function
>> such as    time delay =3D n**(ADC-voltage * m).  This could be done
>> with a lookup table.
> Don't know if the time delay needs to be 'infinitely' variable. The pot
> can be broken down into segments to act more like a rotary switch.
> eg 0-100 =3D 1 sec, 101 - 200 =3D 2s etc or whatever increments are
> most useful
>
>> Instead of a potentiometer , you could have a pair of buttons
> Perhaps a pushbutton to over-ride the pot, or have the highest value
> (eg > 1000) of the pot =3D never
>
> Joe
>

--=20
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