[sdiy] D Flip Flip start-up state

[Scott Gravenhorst]

Dave Magnuson <resfreq at hoohahrecords.com> wrote:
>Hi List,
>
>I'm currently designing a circuit that uses a momentary puch button to
>create an A/B switch.  The design is simple, and I'm sure most of you have
>seen or used it before:
>
>A momentary push button is debounced and shaped to create a clean logic
>pulse.  This pulse is then sent to a D flip-flop (basically a divide-by-two
>circuit).  The flip flop is wired with the incoming pulse to the clock
>input, the Q output goes to the next circuit stage (relays in this case),
>and there's feedback from "not Q" to D.
>
>Well, here's my question:  I know that during power up I'll end up with a
>random setting.  Is there any way to force the flip-flop to always reset to
>the same position when powered up?  I don't need it to have "memory" where
>it always defaults to the last position used before it was turned off... I
>just want it to always come on in the same position.

I would use a Schmitt trigger gate to watch a capacitor charge through a resistor that
is tied to logic +V.  The time constant of the RC should be longer than the power up
time.  The output of the Schmitt trigger is then differentiated (series capacitor) to
turn the edge it produces into a short pulse which is then applied to either the
direct set or direct reset input of the flipflop depending upon which state you'd like
to start up in.

>Since the output from the debouncer should be logic low at start up
>(assuming no one's touching the button), I'm wondering if I could simply
>pulse the flip flop's reset line at power up, but I'm not sure of the best
>way to generate this one-shot without too much circuitry.  Any ideas?
>
>Thanks everyone,
>
>Dave Magnuson
>
>Resonant Frequency:
>resfreq at hoohahrecords.com
>http://www.hoohahrecords.com/resfreq/index.html
>
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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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