[time-nuts] precision timing pulse

Chris Albertson albertson.chris at gmail.com
Tue Nov 15 17:30:02 EST 2016


So you are already using a micro controller (a PIC)  then

1) for the button input use an unused pin on the PIC.  Connect the pin
to 5V via a 10K resister and then when you press the button it shorts
the pin to ground.    The PIC then starts counting.  A modern uP has
the 10K resister build into the chip.

2) I think your entire idea of using the PIC to divide the 10MHz
reference is non-optimal.  You do not need a lower frequency
reference.  All the PIC needs to do is COUNT cycles.   So when the
button makes contact the PIC raises a pin (the ARM signal) and then
starts counting the 10MHz cycles.  When the count reaches 100,000,000
then it takes the ARM signal down and waits for the button to be
pressed.  I just made up the 100M number use whatever value gives you
the correct time interval.  This is a MUCH simpler job than dividing.

3) Why use a large and expensive counter if you already have a micro
controller?   The PIC, while it is counting the 10MHz reference could
also count the pulses from your experiment and output the count after
the time interval is reached.  But maybe you need the input amplifiers
and comparator on the counter?

In short all you need is any reasonable micro controller and MOST of
the single board development systems would already have a button and
the pull up resisters.  Don't bother using a bare PIC chip, it is
cheaper and faster to buy a development board that has all the small
passive parts and is connectorized with 0.1 header pins

On 11/15/16, Chuck / Judy Burch <ciandjaburch at gmail.com> wrote:
>
> I'm building a laboratory scintillation counter that uses my HP 5335a
> counter as a read-out.  The FREQ mode gives only approximate results
> (maximum gate time is about 5 seconds).  The TOT mode counts pulses for
> an arbitrary time that can be set using the "external arm input" on the
> rear panel.  So I need a timing pulse (of either polarity) of known and
> adjustable width (time).  One way to do this is with a PIC frequency
> divider taking the counter 10MHz output down to 1 PPS following that
> with two or three ripple counters to get a 1 - 5 - 10 - 50 ...
> sequence.  That I understand.
>
> How do I get a pulse to start with a push button and then stop for
> example 500 or 10,000 seconds later?
>
>
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-- 

Chris Albertson
Redondo Beach, California


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