Can you name this tune?

[thergen at svn.net]

There are usually the equivalent of diodes on inputs that provide esd
(electrostatic discharge) protection.  These diodes try to prevent the
input from going above vcc or below ground (by more than the voltage drop
across a diode).  Above vcc means above pin 28 and below ground means
below pin 14 because this is where one side of each diode connects to.
So, you can attempt to power the eprom through the inputs because the
power will find a path through the diodes. This has a better chance of
working if the eprom is driven with cmos drivers (like a 68hc11) since the
outputs tend to go close to vcc when driven high or close to gnd when
driven low.  An input may even be tied high or low (vpp on the 27256?).
Don't forget about vpp, ce and oe when considering if there is at least
one input high and one input low.                                              

The above is not intended to be a definitive answer of why it works, but
it's my guess.

My eprom programmer uses ttl drivers and vpp is not tied high (it can't be
tied high if it supports a 27512).  Therefore, I would expect problems
trying to read an eprom with the vcc and/or gnd pins cut.

Hope that helps.

Tom


On Mon, 26 Jul 1999, Shannen Durphey wrote:

> Teller.John at orbital.com wrote:
> > 
> > Pin 28 on an EPROM is usually VCC (+5V).  The part will not function properly
> > without it! Are you sure the pin didn't get broken off somehow?
> > 
> > --- John T.
> > 
> > >TPIS puts happy faces on their proms and paints them gold.  I bought a TPIS
> > >level 2 chip.  Word of caution.  Unless it's an early chip, the glue the lid
> > >on with silicone and cut pin 28 so you can't read the chip.
> > >I read it anyway.  I WIN!
> > >Mark
> 
> Yup, they glued the lid on, and cut two pins.  First I wasted a bunch
> of time verifying that I couldn't read the bin.  The I decided that
> the glued cover was there to prevent me from looking underneath, so I
> looked underneath. ; )  First find was a cut pin.  A small jumper made
> from a safety pin allowed reads, but no two reads were alike.  More
> time spent cleaning the cut pin and the jumper, then went back to
> digging away the rest of the silicone and voila! a second cut pin. 
> After fashioning another jumper, I got a good repeatable read.  I
> removed the jumpers, put eyebrows on the happy face, glued the cover
> on and sent the car on it's way.
> I won too.  The prom _did_ work with these two pins cut, whichever
> ones they were.
> Shannen
>