Re: gEDA-user: pcb crooked traces

[Andrew Poelstra <asp11@xxxxxx>]

On Fri, Oct 08, 2010 at 11:46:39AM +0200, Armin Faltl wrote:
> Gabriel Paubert wrote:
> >Really, the inch is by definition 2540µm, not the other way around
> >since over 50 years ago.
> As far as I know, 1" = 25400um, but I see your point ;-)
> 
> The only practical consideration I see is, that the internal unit of PCB
> allows handling with integer-arithmetic (makes comparisons a lot
> faster and safer than floating point).
> Assuming 32-bit signed numbers with 1/100mil this gives:
>    254nm resolution and +-545.46m coordinate range
> 32-bit signed and 1nm gives:
>    1nm resolution ;-) and +-2.147m coordinate range
> 
> I don't know, if pcb really uses fix-point arithmetics, but even if
> not a reasonable internal unit has some importance. AFAIK with
> floating point, the average internal number should be around 1.
> 
> HTH, Armin
>

I don't think we could reasonably use floating-point. There is no
room for rounding error when designing tight areas of a PCB.

As for board limitations, I think that if you are designing boards
bigger than 2m (and cannot make a small board then scale the gerbers
after-the-fact), chances are you've got a 64-bit system.

Of course, then we need to worry about file-format compatibility
between 32- and 64- bit systems...

Suppose we stored a scaling factor in the .pcb files, of x10, x100,
x254, etc? Then we could use nanometer precision by default and go
bigger if we need a bigger board.


Andrew



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