| 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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