[sdiy] Funky flip-flops

[jays at aracnet.com]

Reset sounds funky.

1) Holding the reset lines low has the FF in a constant state of reset. They are active low and should be tied high (reset to +V) with a 1K-3K ish resistor. Or better yet put a cap in there between reset and ground (with the resistor in there) to form a RC that slowly charges up to V+ providing a power on reset. Also if you aren't using the resets individually you should just tie them altogether.

If I were to guess I'd say there might be some interaction between the PS and the 10K reset resistors not providing a 'weak ground'.

2) If you tie stuff to ground don't use a resistor.

3) Put some sort of RC on the switches going to CP.

4) Put a pull up (1K-2K) on the ~set lines. It's active low to.

Jay S.


Karl Ekdahl <_nial_ at yahoo.com> wrote :

> Hi all, i've got a strange problem. I have 8
> flip-flops (4 * 7474) which all share a common ~SET
> line and each have it's ~RESET tied to ground trough a
> 10k resistor. Each flip-flop is connected to itself
> via ~Q -> D and each is set/reset trough a push button
> connected to CP. All Qs are connected to one LED each.
> So, pushing the first button sets/resets the first
> flip-flop. Pushing the 8th button while the first
> flip-flop is set resets the FIRST one and sets the 8th
> one. Pushing the 8th resets the 8th flip-flop and then
> pressing the 1st sets BOTH flip-flops. All the
> flip-flops reacts like this with eachother in all
> kinds of strange patterns. The only way they possibly
> COULD interact with eachother should as i understand
> it be 
> 
> a) the comon SET
> b) trough the RESET resistors to ground (?)
> c) trough the 74245 that delivers the signals from the
> pushbuttons.
> d) powersupply noise (?)
> 
> a) could probably be ruled out since they bot SET and
> RESET eachother. b) and d) is the same i guess and i
> have a pretty huge powersupply feeding only a couple
> of TTL chips, so that seems unlikley. I had
> powersupply problems earlier but fixed that. c) the
> 74245 seems to give perfect result while looking at it
> with my multimeter, so i don't think so...
> 
> I'm stranded, anyone?
> 
> btw. i might as well thank everyone for their help
> with various things the last couple of weeks, i'll put
> some pictures and stuff of what i'm doing somewhere in
> some time..
> 
> Karl