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'[OT]: Magic CMOS chips'
2001\10\08@153141 by Wade Carpenter

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Hi Everyone,

Just thought I'd share a funny experience from the lab.

We needed a 15-bit counter from parts that we have available to us in the lab, so we took two CD4024 7-bit counters and a dual JK Flip flop to do the job.  
Well, we laid them all out on a breadboard, conencted all the power to the right places, fired it up and saw that the output from the last of the CD4024's was pretty bad.  It was counting correctly and everything like that, but only after we put some fairly large capacitance across power.  
Anyway, after a bit of playing around we realized that this was one of those breadboards with the split power rails, so we weren't actually powering the chip up!  Needless to say, once we gave it power, the output seems to be a lot nicer.

The strange part is that the thing was still counting properly, power or not!  If only every chip ran like that!

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2001\10\08@210257 by Matt Pobursky

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The counter chip was being powered by one (or more) of the other counter chip
outputs connected to it's inputs. The CMOS input protection diode connected
from the input to VDD will conduct when forward biased, providing a postive
rail one diode drop below the driving logic level.

PIC's will do the same thing when an external voltage is applied to a port pin
while no power is applied to the PIC. If the external source is strong enough,
it will power the PIC (or in your case, the counter)...

Veteran PIClister's have heard the question "The power is off -- WHY is my PIC
still running?" many, many times! ;-)

Matt Pobursky
Maximum Performance Systems


On Mon, 8 Oct 2001 12:29:51 -0700, Wade Carpenter wrote:
{Quote hidden}

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2001\10\09@172251 by Peter L. Peres

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<grin>. I try to favor CMOS 4xxx series logic to anything else when I can.
There are several commercial designs of interfaces etc that use what you
have discovered. The chips run on power from the H levels on the clock or
data input,  it being rectified into the supply rail through the upper
(towards Vdd) input protection diodes. Since at low clocks these chips
draw microwatts this works pretty well, as long as there is enough
decoupling to supply during transients.

Also a CMOS series flipflop with a simple 0.1uF decoupling and a diode to
separate the supply would make a 1-minute persistent memory iff the output
is not loaded (driving current) and the reset state is high, not low. Both
can be arranged easily in a circuit (this is relevant to the one bit
memory thread).

Peter

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2001\10\09@223250 by myke predko

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Hi Wade,

You can actually power a PICmicro in exactly the same way - leave the Vdd
pin open on a PIC16F84 and drive a current through one or two I/O pins set
to "input".  You'll find that the oscillator will start up and the chip will
run quite well (although logic output levels will be lower than the input
pin voltages).

myke
{Original Message removed}

2001\10\09@224847 by Wade Carpenter

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Hehe

It has to be the funniest mistake I ever made, because everything still
worked (almost!)

It could have gone undeteced for weeks!
{Original Message removed}


'[OT]: Magic CMOS chips'
2002\01\07@113850 by David VanHorn
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You were powering it through it's protection diodes.
This works, until the inputs all go low, or you need a spike of current due
to a lot of gates changing states


{Original Message removed}

2002\01\07@122706 by David VanHorn

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>The strange part is that the thing was still counting properly, power or
not!  If only every chip ran like that!

You were powering it through the protection diodes.
As long as one input is high, and you have ground, they will run. However,
you were in danger of CMOS latchup, and as you noted, the outputs are pretty
bad.

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