Re: gEDA-user: Spooky Action . . at a distance!

[Mike Jarabek <mjarabek@xxxxxxxxx>]

Hi,

   Most CMOS devices like your 'HC595 have protection diodes connecting 
the inputs to the power rails.   I noticed that some of your pins are 
directly connected to the power supply.  Current could flow through 
those pins and into the device, leaving enough margin for the guts of 
the chip to work. (Eg. the voltage inside the chip was Vcc - (somewhere 
between 0.3 and 0.7) V.)

   O.T. but, as a side note if you intend to manufacture these boards 
and test the board with In Circuit Test (ICT) you should probably use a 
pullup on each of the inputs you wired directly to Vcc.  Eg. something 
like a 1K or 3.3K resistor.  If you have to wire to ground use a 100 or 
110 Ohm resistor.  The ICT machine can overdrive these resistors to test 
all the I/O's on the chips...

Mike

Samuel A. Falvo II wrote:

> OK, I just had the *weirdest* experience of my life.
>
> Spooky.
>
> I am working on wiring up the IPL circuit as documented at
> http://www.falvotech.com/projects/kestrel/repo/8k/prints/ipl-port_3.pdf
> .  I plugged power in, and I ran the software on my Linux box to
> program it (wrote it myself; sources coming soon).  I probed around
> with the o'scope, and everything was working.
>
> Perfectly.
>
> Shortly afterwards, I noticed that Vss of U301 was not hooked up to
> anything!!  Vdd was connected to +5V, as expected.  And yet, it worked
> fully.
>
> Anyone else ever have something like this happen?  I know this is
> pretty much just a fluke, but I'm wondering *why* it worked?
>
> Anyway, I hooked Vss back up, and the circuit operation didn't change
> at all.  :-)
>
>  

--
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                             Mike Jarabek
                               FPGA/ASIC Designer
http://www.istop.com/~mjarabek
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