[Arm-netbook] HDMI High-Frequency Layout: Recommendations

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Mon, Aug 28, 2017 at 10:13 PM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> On Aug 22, 2017, at 08:13, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
>>
>> On Tue, Aug 22, 2017 at 2:43 PM, Richard Wilbur
>> <richard.wilbur at gmail.com> wrote:
>>> How much intra-pair skew do we incur at each of those bends?
>>
>> very little.  it's a 45 degree bend in each case so.... can probably
>> work out the maths... 15mil separation...
>
> Between traces of the same differential pair (intra-pair) I would have expected 5mil separation.

 sorry i was referring to outer-edge to outer-edge so 5 trace + 5 gap + 5 tracee

> I'm not entirely sure how PADS does the trace length calculation:

 no idea.  it's most likely to be down the middle.

> 2.  Is it measuring the centers of the 5mil wide traces?  If so I'd expect the intra-pair skew to be 10mil.

 true!  this i would expect.

>> so in the attached diagram those traces i put right at the bottom
>> will be overwritten by about... 1.2 mil to make up to the 11.84
>> board-to-copper clearance.
>
> Does that mean that flood fill will cover out from 13mil
> off the board edge (as noted in the diagram) up to the
> 11.84mil board-to-copper clearance?

 remember i mentioned that the flood-fill has some "curvature" rules
on it: if the radius (a parameter somewhere) is too small the
flood-fill won't go in there.

example attached: see green arrow, the track "jigs" in just at that
point.  grey is the flood outline btw.  at that point the radius of
the grey flood-fill just below the green arrow is too small, so the
flood-fill refuses to go into the gap between the board edge and the
track with the "jig" in it.

so instead what i have to do here is to make that "manual" GND track
11 or so mil wide, and that does the trick.

anyway *apart* from those special circumstances, what will happen in
the case you refer to is that PADS will create a tiny bit of GND
copper (13-11.84 = 1.16mm) wide just *below* that VIA's outer edge,
including covering the via itself.

so.. yes!  that's actually shown in the 2nd picture.  i selected
(highlighted) the VIA, and you can see how the flood-fill outline
(grey) overlaps the entire VIA (outer extent of which is 13mm from the
board edge) but the flood comes *no closer* than it is told to, which
is 11.84mm.

basically the VIA - which is a GND via - is totally irrelevant to the
distance / extent that the flood fill goes to the board edge.  now if
that VIA was NOT a part of the GND net *THEN* the flood fill would
avoid THAT via (by the amount set in the Design Rules).  but if it's
the same net, flood fill "ignores" it in effect (and just floods over
it as if it wasn't there).

l.
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