Electronics > Beginners
74HC390: Ripple Counter is Counting Rubbish
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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