[sdiy] D Flip Flip start-up state

[Ray Wilson]

Hi Dave

I would give the reset a nice long pulse during start up. Here is how. Use a
CD40106 hex schmitt inverter. Place a 10uF cap at an input along with a 1
Meg to V+.  When the power is off the resistor will discharge the cap into
the circuit. When the power is applied the cap will have a low level which
will cause the CD40106's output to be high which will hold the flip flop
reset. After the cap charges via the 1 Meg to 2/3 of the supply voltage the
output of the CD40106 will go low which will not affect the flip flop's
operation after that. If you want the cap to discharge very quickly when
power is removed then in addition to the 1Meg to V+ place a diode (anode to
the CD40106 input the cap is on, kathode to V+). That way when power is
removed the cap will discharge much quicker into the circuit via the diode.

Hope this helps

Ray

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Dave Magnuson
Sent: Sunday, August 31, 2003 10:41 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] D Flip Flip start-up state


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.

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