[Arm-netbook] Plug computer and FreedomBox-related devices

[Hrvoje Lasic]

On Wed, Sep 16, 2015 at 3:41 AM, Luke Kenneth Casson Leighton <lkcl at lkcl.net
> wrote:

> On Tue, Sep 15, 2015 at 6:41 PM, Hrvoje Lasic <lasich at gmail.com> wrote:
> >
> >
> > On Tue, Sep 15, 2015 at 7:22 PM, Christopher Havel <
> laserhawk64 at gmail.com>
> > wrote:
> >>
> >> On Tue, Sep 15, 2015 at 1:18 PM, Luke Kenneth Casson Leighton
> >> <lkcl at lkcl.net> wrote:
> >>>
> >>> [...] even i was unable to move the micro-desktop board (which is
> >>> only 4in x 4.5in) forward because i had designed it as a 4-layer PCB -
> >>> costs are around $400 for qty 5 4-layer prototype boards on a 3 week
> >>> turnaround  by complete contrast, a 7-day turnaround for qty 5 2-layer
> >>> prototype (bare copper) boards with larger vias is around $40 for qty
> >>> 2, and around $100 for qty 5.
> >>
> >>
> >> Imbecile question: is it at all reasonably possible to redesign the
> >> micro-desktop board to be a 2-layer board?
> >>
> >
> > If board have high speed design it is very unlikely that you can do it
> > properly (because you have to be careful about routing to ground, you
> have
> > many lines that you have to match etc etc and when you have only 2
> layers it
> > is difficult to keep it all correct). In theory maybe it could be
> possible
> > but it is a hell of a work and then you risk a lot that PCB will not be
> > good.
>
>  i took a look at a gigabit ethernet board that phil kindly sent me a
> while back.  the layout of the differential pairs was absolutely
> fascinating [and the board had, obviously, passed FCC tests].
>
>  the layout involved putting ground vias exactly... something like
> 20mil from the differential pairs, spaced out at exactly something
> like... 100mil, right the way *both* sides.  there were no components
> permitted either side for some distance either.  there were no vias in
> the actual differential pairs, either.
>
>  there were twenty sets of differential pairs like this - all with
> exactly the same very very clearly and meticulously laid out
> arrangement, with the spacing between each differential pair also very
> meticulously laid out.
>
>  so it can be done.
>
>  in the micro-desktop board, however, the actual distance that the USB
> differential pairs has to travel is well under 1cm.  i am arranging
> the connectors *directly* in front of where the signals come out.  i
> have also deliberately arranged the EOMA68 interface so that the pairs
> come out directly and do not require a via to "cross over" each other.
>
>  so it is much less of a concern than might otherwise normally be.
> these aren't 10cm traces, where EM radiation would definitely be a
> major concern, they're literally 1cm long, and i intend to surround
> them with ground vias.
>
> of slightly more concern is the VGA interface (RGB/TTL being converted
> to analogue), which will be operating at around 75mhz (or so).
> however that's not 480mhz, so i am not hugely concerned.  i am again
> getting in as many GND vias as can fit, and keeping the traces very
> very short.  the buffer ICs actually straddle the PCMCIA interface on
> the other side of the board, so that the RGB/TTL signals can, with
> vias, go left or right, routing as appropriate, maximum trace length
> about... 3mm.
>
>
anyway, absolutely amazing work to fit it all like you described.


> l.
>
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