Re: gEDA-user: Introduction and some questions/thoughts on gEDA/gaf...

[Dan McMahill <dan@xxxxxxxxxxxx>]

> Not all glue logic devices are created equal. There are devices made by
> different manufacturers that have the same part number that are *not*
> interchangeable under all conditions. The pinout is the same- Everything
> appears to be the same when you look at the data sheet until you look at the
> diagram of the internal logic of how the chip works. In essence, the structure 
> of the internal logic of a given device that is made by multiple 
> manufacturers differs. That is,  Unless one manufacturer has granted a 
> license to another manufacturer to make a device using their logic and 
> process. So, tags don't work - They just cover up the problem. 
>
> Also, this issue goes way beyond glue logic into many other families of 
> devices.

To add to what Marvin said, I have first hand seen where two vendors of 
a particular comparator were not the same because the ESD protection was 
significantly better from one vendor than another.  Learned the hard way 
when purchasing "saved some money".

This isn't limited to semiconductors by any stretch.  Take an 
electrolytic capacitor of a particular capacitance and voltage rating. 
Now see how many variations in ESR (causes internal heating if you're 
using this in a switching power supply app), quoted lifetime, 
temperature at which that lifetime is quoted at, etc.  Of course the 
symbol can always be the same.

What I've seen in at least one large company is the CAD libraries are 
all organized by internal part number anyway.  In other words, if you 
want a "1206 size, metal film, 1/8W, 1% 4.99k ohm resistor", you simply 
instantiate a "192837.sym" which you know because you have some silly 
notebook on your desk listing all the common passives you use and you 
have the internal part number database open to search for the 
semiconductors.

Something which is missing is a means to have a component have 1 extra 
level of hierarchy though.  I'd like to be able to create a single, for 
example, NPN symbol.  Then create a library of components which all use 
that symbol but each element in the library also then contains 
information on what package to use, a mapping from package pin to symbol 
pin (package #1 -> symbol 'e', package #2 -> symbol 'b', package #3 -> 
symbol 'c').  This way I don't end up copying the actual graphic symbol 
50 times.  It would also be nice to be able to associate some 
information on how it should netlist for different targets.  That might 
make it easier to truely support multiple simulators as well as pcb.

I think at some point gschem may need to start storing elements both by 
the symbol name and the library from whence it came.  Currently the 
library search path matters, and its only a matter of time I think until 
this starts to cause grief.

-Dan