Re: gEDA-user: How to deal with single/dual parts?

[Bill Gatliff <bgat@xxxxxxxxxxxxxxx>]

Bill Gatliff wrote:
> * a schematic "symbol" represents some or all of a "component"
> * a "component" might satisfy the functionality indicated by more than
> one symbol
> * a "component" comes in one or more "footprints"
> * "footprints" are used by more than one component
> * schematic hierarchy symbols are just collections of "symbols"
>   

More brainstorming.

Looking at things from a different direction, a component "provides" one
or more symbols depending on the component.  Some of those symbols might
meet a "requirement" described in a schematic diagram, in which case you
can check both symbols off.  Making a component selection means that
_all_ the symbols come into your design, however, and if you forgot to
deal with the power pins then they'll look like symbols with floating
pins if you represent them in a schematic diagram.

A symbol is just a graphical entity, and each one that appears in a
schematic diagram defines a requirement that must be satisfied by a
component.  A symbol:

two-input-nand.symbol:
    "symbol=two-input-nand"
    "input=A"
    "input=B"
    "output=C"
    (verbage to describe what the symbol looks like on the screen)


A component supplies symbol instances to a design described by a
schematic.  They also have to provide enough information to associate
the symbol's "pins" to pins on a footprint.  Each time you specify a
component, ALL of the component's symbols become a part of the
machine-generated version of your schematic diagram:

sn74x2x00.component:
    "component=sn74lvc2g00"
    "component=sn74lv2g00"
    "component=sn74act2g00"
    "symbol1=two-input-nand A:A1 B:B1 C:C1"
    "symbol2=two-input-nand A:A2 B:B2 C:C2"
    "symbol3=two-input-nand-power V:VCC G:GND"
    "footprint=soic8 A1:1 B1:2 C1:3 A2:4 B2:5 C2:6 VCC:7 GND:8"
    "footprint=dip8 A1:1 B1:2 C1:3 A2:4 B2:5 C2:6 VCC:8 GND:7" # swap
VCC and GND pins, just for fun


A "footprint" describes the physical layout of a footprint, along with
information to map virtual component pins to physical pin locations:

soic8.footprint:
    "footprint=soic8"
    (text that describes the physical footprint itself, and enumerates
each pin)


A schematic diagram would be just line after line of something like this:

symbol=two-input-nand
    id=EA89-ab32-ifj2-ee89 # machine-generated, unique (uuid?)
    x=1
    y=2
    (other stuff to describe wires, etc.)
symbol=two-input-nand
    id=2aef-789j-ie87-po73
    x=100
    y=200
    (...)


Now things get tricky.  We want to tell the tool what chips we're going
to use, and in some cases which "slot" of the component is utilized by a
corresponding symbol.  Ultimately, you'd produce a file that looks like
this:

component=sn74lcv2g00
    refdes=U1
    footprint=soic8
    id.1=EA89-ab32-ifj2-ee89
    id.2=2aef-789j-ie87-po73

The above says:
    * We have a sn74lvc2g00 with reference designator U1 and footprint
"soic8"
    * U1's symbol1 is taken by the schematic symbol with id=2
    * U1's symbol2 is taken by the schematic symbol with id=1
    * Nobody took U1's symbol3 (now that they know the chip, they need
to wire up the power)

You'd feed your schematic file, component files and component list file
to a tool that would correlate schematic symbols with component symbols,
making sure that the tally for each was the same.  If the two didn't
balance, it would kick out a meta-schematic file with the unused symbols
and then complain.  Now you go wire up your power pins.

Once the symbols and components balanced, a tool could take the above
files and produce the same information that comes out of gsch2pcb today,
methinks.

In the above, I'm not proposing that we change the syntax of any of the
existing gaf files.  I'm just trying to describe the information itself.


b.g.

-- 
Bill Gatliff
bgat@xxxxxxxxxxxxxxx



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