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

[John Doty <jpd@xxxxxxxxx>]

On Nov 19, 2009, at 9:38 AM, Bill Gatliff wrote:

> Peter Clifton wrote:
>> Lets say I have a symbol:
>>     ____
>> ---|    \____
>> ---|____/
>>
>>
>> That might have 4 slots - IE.. I expect 4 of those nand gates in the
>> chip.
>>
>
> ... and I think that might be where the problem suddenly creeps in.
>
> The _component_ defines how many and what kinds of circuits it
> contains.  It also defines what packages the component is available  
> in,
> and how to map the pins of its composing circuits to the pins on the
> package.
>
> The _symbol_, on the other hand, just expresses the designer's
> _requirement_ for a circuit.  The symbol itself doesn't care which
> _component_ satisfies that requirement, it's the developer's job to at
> some point identify which circuit of which component will satisfy that
> requirement.
>
> To give a concrete example, let's say my schematic diagram contains
> three nand symbols, which specifies a requirement for three nand
> circuits.  Some other document would record that I will provide two
> 74ls00 components, which supplies the design with a bucket of eight  
> nand
> circuits plus two power circuits.  A tool that tries to correlate the
> two documents can detect that my requirements are being over-supplied,
> which means that my schematic diagram is missing symbols that  
> correspond
> to the actual number of circuits I'm providing.  Better fix that,
> because some of those circuits are the power supplies for the  
> components
> and the other circuits won't work without them!  I add the appropriate
> symbols to my schematic diagram until everything balances.

Actually, what I'd like to do is provide the tool with a power  
circuit template, and rules on how to use it. Let the tool figure it  
out.

>
> So then I change my mind, and provide just one 74ls00 component.  The
> same tool can detect that now there's an imbalance in the other
> direction, meaning that I need to get rid of one of the power symbols
> and a few of the unused nand symbols that I added previously.  But
> otherwise, the schematic diagram is the same as it was before--- as I
> think it should be.
>
> Now I'm beginning to see the problems with slotting and symbols the  
> way
> we're doing them now: they unnecessarily tie the concept of a  
> symbol to
> the concept of a component, because the pin numbers that we currently
> record in our symbols are also the pin numbers that the component maps
> to the pins of the component's package.  We have munged together the
> concepts of symbol and component in our "symbol" files, but can't seem
> to admit to that.
>
> That's the breakage, methinks.

For a certain kind of flow, yes. But on a hobbyist scale, the way it  
works now isn't so bad, and another layer of processing would be a  
burden. *Any* discussion of how such a thing "should" be done is  
dependent on flow and scale. So the kit should provide the tools to  
do it either way (and other ways, too).


John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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