[sdiy] D Flip Flip start-up state

[harrybissell]

Like Ray said... but

10uF is a rather large cap to have hanging on a CMOS input... I'd add a resistor

(maybe 10K) in series with the input pin.  This is so that if you get a
catastrophic
short circuit on the supply (elsewhere)... that 10uF will try to dischage
through the
input protection diode and might snuff the chip. A 10uF cap is capable of AMPS
of current... but almost any series resistance will spoil the party.

In a modular, where mods are often made... this could save a lot of trouble.

Usually I try to stay below .1uF for timing caps w/cmos circuits... even they
can pack
quite a charge, but the input protection resistance (usually 150 ohms) will
probably
keep you safe.  The added resistor will not hurt, I consider it a $.02 insurance
policy.

H^) harry

Ray Wilson wrote:

> 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