[sdiy] D Flip Flip start-up state

[Magnus Danielson]

From: Dave Magnuson <resfreq at hoohahrecords.com>
Subject: [sdiy] D Flip Flip start-up state
Date: Sun, 31 Aug 2003 12:41:16 -0400

> 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.

Use a D-flip-flop with a RESET input (look at say 74HC74) and use a traditional
power-reset setup (basically a resistor, cap and diode).

Doesn't that fix it?

Cheers,
Magnus