On Sat, Sep 23, 2017 at 8:26 AM, Richard Wilbur <richard.wil...@gmail.com> wrote:
> After spending a couple minutes studying layers 3 and 4, here's what I see: > 1. It looks like there may be a difference in the signal via antipads on > layers 3 and 4 and that would be a way for us to give just that handful of > vias special properties--if need be--although in this case it interestingly > looks like the antipads are larger on layer 4 than layer 3. (optical > illusion?) > 2. The (minimum?) polygon size or line width of the fill looks larger on > layer 3 than layer 4. layer 4 HDMI vias are actually covered by VCC 3v3 flood-filled plane. it's a 5mil clearance to that. i decided i didn't like that, so i made a cut-back in the 3V3 plane so that GND covers it instead. it's *still* 5mil even on that flood fill. so it's just something weird about the flood-fill on layer 3, possibly due to it being a copper pour not a "plane area". don't know. if absolutely necessary i can put in some tracks that split the pairs. > I think the second point, or something along those lines, likely explains the > void on layer 3 and lack of void on layer 4. It looks like if we were to > find and adjust that fill parameter on layer 3, some of your explicit guard > traces might become redundant. (I can see why you added them because the > ground fill wasn't working as expected.) > > It would be nice to change layer 3 to make the signal path more uniform on > the way through the vias but it's not the end of the world if we can't, as > long as layers 2 and 5 resemble layer 4 in the vicinity of the HDMI > differential signal vias adjacent to the A20. i have to use it as a signal layer, so there's tracks running round the back of some of the diff-pair VIAs. > The remaining questions are where do we impose 5mil clearance (by bringing in > the fill), where do we start tapering, and what does the taper look like? > Likewise, but in reverse order, at the other end. yehyeh >>>>>>> Is the closest copper on layer 1, around the A20, 5mil from the HDMI >>>>>>> differential signals? >>>>>> >>>>>> yes. everything's 5 mil design rule. >>> >>> I agree that 5mil is the design rule. The question is, "How close did >>> we actually get?" What I'm referring to as foreign copper is any >>> trace, via, component land/pad, or fill that is not part of the >>> differential pair under consideration. In other words, did we make it >>> from A20 land to via without getting closer than 10mil? 7mil? We can >>> adjust the proximity of ground fill with a manual keepout if we need >>> more space so I'm not too worried about that. I'm more curious about >>> distance to other traces, lands/pads, or vias. >> >> ok - let me re-run the flood fill and do a quick review, starting from the >> A20. >> >> so. layer 1. surrounded, all 5mil. tracks are only 60mil or so to >> the VIAs. didn't do a keepout. all 5mil. >> >> layer 3 (the VIAs) - some sort of curve on the flood-fill, it's 5mil >> but there's a void in the middle. > > Are layers 2, 4, and 5 also 5mil away from the differential signal at the > vias? yes. layer 3 is the only exception. >> layer 6, starts @ 5mil, expands out to 15mil (mostly). exceptions: >> distance to TX2 "long wiggle" is 7mil, distance from bottom VIAs >> along board edge (to TXC), 11.2mil, distance to track *between* the >> VIAs 15mil. distance to GND vias ABOVE the hdmi tracks (TX2), 19mil. >> >> in theory then i could move the entire set of horizontal tracks up >> by... 4 mil... i reeaallly don't want to though as it means redoing >> the whole f*****g lot of wiggles.... argh :) > > Can't select and move? you can... but there are special rules which ensure that 45 degree angles on two adjacent segments are "respected". it gets extremely weird and extremely frustrating. > Since it is such a long section it would be beneficial to move the traces. argh. i kinda reached that conclusion :) what i can do to some degree is manually enter values (adding 4mil up and 4 mil left/right) so that there's less to redo by hand. > So if we bring in the keepout at 5mil on layer 6 and taper it slowly to 7mil > by the point we get to the TX2 wiggle which exhibits 7mil clearance. To make > this work we have to start the pairs off around 5mil inter-pair spacing and > then spread them as we taper the keepout. I realize this is more complicated > than what I first described. it's too much. i can just about manage adding 4mil manually to every single one of those long straights, moving them up from the 11mil clearance to the bottom board-line VIAs to 15mil, thus taking 4mil off that 19mil clearance and resulting in 15mil there as well. we don't have *room* for 7 mil inter-pair spacing. if i've misunderstood, do let me know. >> yehhh there are so many GND vias at the ESD end i'd question its >> effectiveness... > > You'd question the effectiveness of what? The ESD component? The taper? putting in a taper at the end is significantly disrupted by the presence of non-removable VIAs. you can *add* a taper... but then the VIAs (which cannot be moved) are *already* within about 5mil or 7mil of the tracks. what *would* work is bringing the taper in *BEFORE* the ESD components. it also coincides with the double 45-degree bending of the group of tracks, so is still a bit... dodgy. see this picture for reference: http://rhombus-tech.net/allwinner_a10/news/eoma68-a20-275-layer6-hdmi.jpg basically there's no point in tapering *after* the ESD components because the GND vias are already closer than the taper would bring GND in. l. _______________________________________________ arm-netbook mailing list arm-netbook@lists.phcomp.co.uk http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook Send large attachments to arm-netb...@files.phcomp.co.uk
