Electronics > Beginners

74HC390: Ripple Counter is Counting Rubbish

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Darkwing:
Hi folks!

I'm currently experimenting with a CD74HC390 Ripple Counter. I want to use it to simply divide a pulse signal. Here's the breadboard:



It seemed an easy task at first, but I found out that it is not so ...   |O

Why do I get this from the outputs:



When the datasheet says, that I should get this:





It's messed up completly.  :(
The QA stage does invert (?!) my signal, instead of dividing it by 2!  :-BROKE
The following stages QB and QC are not even close to a recognizable pattern.

What could be wrong?  :-//

Is it somehow necessary to "stabilize" the IC, that it can work correctly? I already found out, that a 0.1µF cap across VCC and GND does a little good to the signal quality.

Can someone give a hint?


Thanks in advance!   :) :)




EDIT: You can jump right to this message, to see more pictures and a solution.

sarahMCML:
Hi,

Your layout looks fine, but you may have a bad connection somewhere. The most likely reason though is because the other section of the counter has its inputs left in an undefined state. Try tying the two A side clocks and its reset to 0V, and it should eliminate any spurious interference from that side.

Regards,

Sarah.

schmitt trigger:
HC logic is very fast. Sub microsecond noise pulses, completely invisible at your ultra-slow timebase, can cause spurious clocking.

It has happened to me.

EEVblog:
HC has a maximum input slew rate, so make sure your inputs are nice and fast edges.

EDIT: Yep, your clock pulse is coming from an opto-coupler, that will be a creating a slow positive edge. Try reducing your pullup resistor value for starters.

floobydust:
I never leave unused 'HC logic inputs floating, like pin 1,2,4.

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