CD4017 chip output latching problem

HI all,

I've been having problems with a timer circuit I'm building. It uses a chai n of seven CD4017BE decade counters. The first in the chain gets clock puls es from a 555 running at about 10Hz. I take the last output from this chip (puts out one pulse for every 10 input pulses) and feed it to the input of the next chip where the same thing is done and so on so the pulses get time

-divided by 10 at each stage. All's fine up to decade 4, then something odd happens. Instead of just pulsing, the output goes high and remains high un til the next pulse comes along and toggles it back to low, so this stage's output is high for far too long. For this prototype I'm using rat's nest on PCB construction and believe I'v e paid proper attention to grounding and decoupling. Funny thing is, if I t ransfer the components over to proto-board, the problem disappears. Do thes e symptoms ring a bell with anyone? Is the 4017 particularly layout-sensiti ve? It's driving me nuts.

TIA

Reply to
orion.osiris
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ain of seven CD4017BE decade counters. The first in the chain gets clock pu lses from a 555 running at about 10Hz. I take the last output from this chi p (puts out one pulse for every 10 input pulses) and feed it to the input o f the next chip where the same thing is done and so on so the pulses get ti me-divided by 10 at each stage. All's fine up to decade 4, then something o dd happens. Instead of just pulsing, the output goes high and remains high until the next pulse comes along and toggles it back to low, so this stage' s output is high for far too long.

've paid proper attention to grounding and decoupling. Funny thing is, if I transfer the components over to proto-board, the problem disappears. Do th ese symptoms ring a bell with anyone? Is the 4017 particularly layout-sensi tive?

I should just mention that if you're one of the people that suggested tips on the previous thread in relation to this issue, I have tried them all to know avail. :(

Reply to
orion.osiris

I don't know what is going wrong - the 4000 series cmos is pretty much trouble free and so is the 4017 (disregard the guy who said the 4017 causes glitches - it does not. A 555 might)

Reply to
David Eather

of seven CD4017BE decade counters. The first in the chain gets clock pulses from a 555 running at about 10Hz. I take the last output from this chip (puts out one pulse for every 10 input pulses) and feed it to the input of the next chip where the same thing is done and so on so the pulses get time-divided by 10 at each stage. All's fine up to decade 4, then something odd happens. Instead of just pulsing, the output goes high and remains high until the next pulse comes along and toggles it back to low, so this stage's output is high for far too long.

paid proper attention to grounding and decoupling. Funny thing is, if I transfer the components over to proto-board, the problem disappears. Do these symptoms ring a bell with anyone? Is the 4017 particularly layout-sensitive?

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If you're using a bipolar 555, then it's _mandatory_ that you connect 
about a 100nF low ESR cap directly across pins 1 and 8 in order to 
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Reply to
John Fields

Well, I did read that the 555 is a bit of a current hog during switching, s o I've used a 100uF electrolytic across pins 1 & 8 of it and hoping that - whilst not optimal - will do. Tomorrow I'll make up another pcb with a diff erent trace layout and see if that cures it.

Reply to
orion.osiris

I've used a 100uF electrolytic across pins 1 & 8 of it and hoping that - whilst not optimal - will do. Tomorrow I'll make up another pcb with a different trace layout and see if that cures it.

Use 100 _nf_ not 100 _uf_

Ed

Reply to
ehsjr

I've used a 100uF electrolytic across pins 1 & 8 of it and hoping that - whilst not optimal - will do. Tomorrow I'll make up another pcb with a different trace layout and see if that cures it.

I second that motion!

a 100nF polyester or ceramic capacitor will have a much lower resistance than an electrolytic capacitor, and that's important for decoupling power supplies

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Reply to
Jasen Betts

Yes, not to mention much lower parasitic inductance to boot! Perhaps a 100uF electrolytic AND a 100nF ceramic in parallel would be the best solution? BTW, have now discovered that the inputs to each 4017 stage must have pull-down resistors. Didn't know that; could be the source of the problem. I'm going to carry out the necessary mods and try it out again. Thanks, all.

Reply to
orion.osiris

electrolytic AND a 100nF ceramic in parallel would be the best solution?

resistors. Didn't know that; could be the source of the problem. I'm going to carry out the necessary mods and try it out again.

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Reply to
John Fields

electrolytic AND a 100nF ceramic in parallel would be the best solution?

pull-down resistors. Didn't know that; could be the source of the problem. I'm going to carry out the necessary mods and try it out again.

Sorry John. I do try to but I'm using the Google Groups interface and it's rubbish. I really shout re-install Agent when I get a minute!

Reply to
orion.osiris

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