--- crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
On Fri, Aug 4, 2017 at 3:24 AM, Richard Wilbur richard.wilbur@gmail.com wrote:
HDMI Layout Notes for EOMA68 Cards by Richard Wilbur Thu 3 Aug 2017
Recommendations for this Layout
Source (processor) end:
Could we shrink the via pitch between constituents of a differential pair (bring the vias of a differential pair closer to each other)
maaayyybeee? PADS does the distances automatically, so it would involve manual editing (and second-guessing of the automated rules / best-practices for diff-pair routing in PADS)
and combine the anti-pads into an oval shape on each layer? This reduces fringing fields and thus parasitic capacitance.
https://e2e.ti.com/blogs_/b/analogwire/archive/2015/06/10/differential-pairs...
ok found it... hmmmm yeah i can see how that would work.
ok see attached little image: turns out that the GND copper flood-fill clearance is enough to *automatically* create the equivalent of what you're referring to.
I like the ground vias close to the signal vias between differential pairs. It would be lovely to get a ground via close to the via on HTX2P and possibly move the one between HTX2N and HTX1P closer to the signal vias (if the signal vias of pairs can be moved closer with combining the anti-pads).
yeah i think i can do the one on HTX2P... but it involves: *deep breath*
moving HSCL and all those other signals further over
inverting the XTAL-IN and XTAL-OUT signals so that one of them goes the *other* side of its BGA pad
moving and re-routing the PWM and EINT-0 signals (which are too close anyway) with those vias, to the XTAL signals
possibly routing HSDA round the *back*... no that takes it past the diffpairs... HHPD routing *right* instead of down... that would be okay.... it would go past the USB diff-pairs though.... i think i can tolerate that..
What is the intra-pair skew from the processor lands to the first signal vias? I wonder if we could move the vias on the short lines a little further from the processor and make up some of the skew in that segment before we leave it?
yes i was considering that - maybe just staggering the vias so for example HXTX1N and P are inverted as to how they really should be.
Sink (connector) end:
Same thing for differential pair vias--HTX0 and HTX2--it would be lovely to shrink the via pitch and combine anti-pads (if possible). Again, I like the ground vias close to the signal vias HTX2P, HTX2N, and HTX0P. It would be lovely to be able to either put a new ground via closer to the signal via on HTX0N or move the one on the ground shield trace closer.
it's virtually impossible to get anything in there, because of the three Rclamp0524p components (anti-static protection).
i'm going to move one of the rclamp0524p's so that it's directly above the other, and it *might* then be possible to fit some GND vias in there.
also i realised that the path is shorter to the DC3 connector because of the via staggering, so i will have to put a small "wiggle" into the shorter path right at that point. why? because the signals should be properly matched right up to that point.
I like what you were showing on the video with the signal vias at the connector lands: putting a neck on the trace between the via and the land should dampen the spirits of the solder but not the signals.
yehyeh.
If we could reduce the signal via pitch by combining anti-pads at the connector, we might be able to move the HTX1P and HTXCN signal vias to the other side of the lands next to the other side of the differential pair, thus equalizing the skew on the segment between the ESD chip pads and the connector pads.
yehyeh i get it.
nomnomnom....
it might just be doable. i'd have to shrink the size of the two GND pads, 16 and 4, then the vias for 14 and 6 could be moved to the other side then *diagonal* (right).... shrinking the size of 10 as well would allow the existing vias to the right of 8 and 12 to *also* be moved diagonally to the right... changing them to 0302s would give some extra clearance, it's risky but what about this isn't...
If that worked the final touch might be to add a ground via between DC3 pin 10 (GND) and the board edge for return current paths.
Other than that, I would try and move as much of the skew compensation close to the source of the skew as possible.
yehyeh *sigh* i missed that. frack. gonna have to redo the whole frackin lot, one path at a time, so i can make sure each segment is matched. frack!!
I'm not sure what I'm looking at as you mentioned the ground reference planes were solid under the HDMI differential pairs,
yes.
but it looks like they have voids under the signals in the pictures.
no,
Am I seeing a negative,
you're seeing the board pre-flood. when flooding is done it f***s things up in PADS, causes it to be very unstable (especially if you switch it to "invisible" with SPO and PO / PD keystroke commands). also massively increases the file size. also gets in the way as you can't see a damn thing.
also if it was a real-time feature the entire system would grind to a halt as it takes about ten SECONDS to recalculate the flood-fill.
and yes layers 2 and 5 are solid GND planes.
that there are only little strips of conductor in the ground reference plane directly under the high-frequency lines? Neither of these interpretations is very satisfactory, nor do they seem to represent reality.
you may be referring to the little ground tracks i added. these are there because the copper-to-everything-else clearance i set to around.... i think... 7 or perhaps 10mil, so that it doesn't get absolutely everywhere.
however i *want* the GND plane (on 1) to go into nooks and crannies.... reach the parts that other beers can't reach... only way to do that is manually.
Please let me know what can and can't be done and I will adjust recommendations accordingly.
appreciated.
well, let me try the DC3 experiment of moving the VIAs to the other side. that i feel is really important.
l.