[sdiy] 4027 flip-flop weirdness

[Scott Gravenhorst]

Karl Ekdahl <elektrodwarf at yahoo.se> wrote:
>
>> Are you _100%_ sure that your clock is really
>> debounced?  Is it slewing
>> fast enough when it changes state?  
>> 
>> What I would do - use a nice square wave oscillator with an
>> output that
>> conforms to the logic standard to drive your clock
>> input.  Then watch
>> the Q output with a scope, it should be exactly 1/2 the
>> frequency of
>> the input clock.  If that works, then the flip flop is
>> toggling and
>> your debounce needs work.  
>
>I tried the above and put a scope on the Q output of the IC. 
>Here's the interesting thing; it didn't do anything until i 
>probed the CP input with another scope probe - apparently it 
>needed some capacitance in the tens of pF-range using the LFO i 
>tried it with. This doesn't really make any sense to me but maybe 
>it does to someone else. 
>
>So, tried it with a very extreme debouncing circuit; 20K input 
>resistance to a 10uF cap with a 1M to ground. One scope probe on 
>the Q output and one probe on the CP input. Here i could see the 
>charge/discharge curve and basically the Q output just reflected 
>the input of CP - when CP goes HIGH, so does Q, when CP goes LOW, 
>Q will follow. 
>
>I've checked my power lines for both HF and LF oscillation but 
>found nothing, i've put a few caps on but to no avail. 
>
>The parts i'm using are CD4027 from All Electronics, they're 
>supposed to be new but frankly they look kinda old.. (not like 
>pulls but NOS) i've never bought logic chips from them before, 
>perhaps it was a bad idea. 

Hmm.  I've bought stuff over there, but honestly never IC parts.  Does this happen with
both FFs in the package?  

The debounce you describe - why the 10uF?  Is this for a switch?  I think a schmitt
trigger following that might be better...  A capacitor/resistor (if I'm visualizing
what you did correctly) will cause the signal to transit logic levels slowly and when
the input is halfway between the IC's rails, it can "chatter" and do naughtiness.

There _is_ always the chance that the IC is not up to par.  If you can get one from
another source, you can prove that.  I've only gotten them (4027 in particular) from
digikey/mouser/jameco, but have never had such problems.


>> -- ScottG
>> ________________________________________________________________________
>> -- Scott Gravenhorst
>> -- FPGA MIDI Synthesizer Information:
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>> -- FatMan: home1.gte.net/res0658s/fatman/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- When the going gets tough, the tough use the command
>> line.
>> 
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>
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.