[sdiy] Issue with CD4024 Ripple Counter

Tim Stinchcombe tim102 at timstinchcombe.co.uk
Mon Sep 20 19:11:56 CEST 2021


Hi list,

> I give up.  I'm going to put all of this shit away and build a few more
Freak Shifts.

Well it seems overnight I've been a little overtaken by these events, but
last night I started breadboarding a little test circuit for the sole TI
CD4024BE that I have, to see if I could provoke it into misbehaving by
gradually slowing down the clock edge rate, and having gotten so far decided
to carry on regardless. Executive summary: I can't, on the face of it, it
basically 'does exactly what it says on the tin'.

My very simplistic circuit and scope traces attached.

My signal generator is no great shakes, but the square wave it outputs has
rise/fall times of 200ns, so plenty fast enough to not need to worry about
further. Feed this (+/-5V signal) into a simple integrator with a 4k7
resistor in series with a 500k trimpot (R1 just shown as 4k7 in the
schematic); thus the output of this OA is slamming nearly rail-to-rail, and
I can control the slew rate with the trimpot; 2nd OA scales this down by
about half, so the signal swing is about 10V, and the resistor to -12V adds
about 5V of offset so the signal is near enough unipolar 0-10V to
comfortably feed the clock pin of the 4024. I'm triggering off of Q1 out of
the 4024, and the 'divide by two' action is clearly seen.

With the trimpot at zero, so R1 at just 4k7, traces annotated 'fast ...',
the slew rate is about 6.5V per microsecond, i.e. not far off as good as a
TL072 will go, and the clock falling edge time is less than 2us. The chip is
clearly reacting to the falling edge of the clock, and there was absolutely
no hint of the 'CLK' trace 'flipping' as it would have if the chip were
occasionally responding to the rising edge of the input. (Note the changed
vertical and timebase scalings for the 'zoom'ed trace.)

With the pot at 500k+, traces 'slow ...', the situation remained exactly the
same, apart from the more sluggish clock edge: the slew rate is now around
0.15V per microsecond, with a fall time of more than 70us.

So on my sample size of 1, the chip seems to behave as it should, with no
hint of any of the weirdness that David is/was experiencing in his circuit,
suggesting that there is either some odd noise/coupling/whathaveyou issue in
his PCB or it just so happens his TI chips are 'wonky' in some unspecified
way.

David: I'd be more than happy to stick one or more of your chips into this
set-up if you felt inclined to post them to me, but it is so simple a means
of testing it probably isn't worth the postage, i.e. you could probably just
as well lash something very similar up yourself...?!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at timstinchcombe.co.uk
www.timstinchcombe.co.uk


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