On Aug 18, 2017, at 19:54, Luke Kenneth Casson Leighton <l...@lkcl.net> wrote: > > On Thu, Aug 17, 2017 at 5:20 PM, Richard Wilbur > <richard.wil...@gmail.com> wrote: […] > So you have a flood-fill on the bottom layer? > > all layers. > >> Is the flood-fill connected to GND? > > only when it's properly arranged to be so... i.e. when you don't you > get a warning... short answer: yes. > >> Can you set the 15mil copper-to-trace separation as a property of the >> differential traces? > > yyup. i really like PADS for this reason > >> The goal with this 15mil clearance is to space other copper >> in the same plane far enough away to have a negligible effect >> on the differential impedance of the differential pair and by the >> same token negligible high-frequency signal coupling. > > okaaay. i get it. > >> The microstrip differential pair geometry is based on having ground >> plane (may it extend forever ;>) > > :) > >> underneath the traces separated by a dielectric of thickness t. >> (We took that into account in the impedance calculations. > > yehyeh. > >> Actually power and ground are identical from the perspective of >> high-frequency signals so we could have built our microstrip >> differential pair over a power plane--or even moved from one >> reference plane to another. > > ohhh that explains why DDR3 has a big power-plane @ the 1/2 way > "reference" voltage. nice. > >>> there's one place where the diffpairs go past the main power line >>> (IPSOUT) - that's got a 5 mil copper GND separating it at present: i'd >>> be nervous about taking that out. >> >> I wouldn't worry because that 5mil copper GND has >> 5mil spacing on each side, thus ensuring 15mil between the closest >> differential trace and power. That should be sufficient. > > ... need to check it. > >> On the other hand, if I remember correctly the proximity to IPSOUT >> happened because we decided to do significant inter-pair skew >> compensation close to the power circuit. > > ah no: it's always been very close: in this revision i particularly > wanted the vias left of the rclamp0524p to be reasonably symmetrical > and clean, with a straight (diff-paired) path to the rclamp0524p > instead of taking a turn to get to it (as in previous revisions). > > that required a little bit more space, which meant moving IPSOUT's > vias a little bit further over. i could _probably_ move them over a > bit further... > > >> The other thing that we can do if we have a little extra space >> after taking out the intermediary GND shield traces and inter-pair >> skew compensation wiggles is distribute the intra-pair skew >> compensation closer to the sources of intra-pair skew--corners. > > aw poop - changing those is quite a task. there's some bugs due to a > combination of grid snap and push-and-shove in PADS where removing the > long straights means i can't add them back in again. and i need to > remove them because otherwise i don't know how long the traces are > from the vias. what i do is: > > * remove the long sections > * re-add a *short* diffpair section of only about 1mm > * those end up being equal length > * then because the traces aren't complete PADS will tell you exactly > how long they are > * therefore i can now measure them both and... > * therefore i know exactly how much manual "wiggle" to put in the shorter one. > > once the wiggles are done i can re-add the long sections, confident > that the signals will be matched. > > but it's a pain to do! :) > >> Right now you've done a great job of compensating for intra-pair >> skew in the first segment: from CPU lands to first via. > > yehyeh. they're near-identical. > >> Then there are some very significant wiggles when we first get >> to the bottom layer > > yes. intra-pair correction due to wanting to have the 1st layer > traces all the same length. it's nearly... 1.5mm to correct, due to > not just the offset of the vias but also the turn. if i tried to > stagger those first vias the other way (which i tried once) then > there's not enough room to have those 1st trace segments be equal > length... > >> and I don't see any other intra-pair skew >> compensation all the way out to the connector. > > that's because they're all fine... ok i read somewhere that it's ok > to have some intra-pair skew on short lengths between turns. sooOo... > i'm assuming that the critical part is the long straight. sooOOo i > arranged for the wiggles to make perfect length-matching just as each > pair hits the beginning of each long straight. > > now (and i've removed the inter-pair skew in the current revision) > what i *haven't* done is add in any inter-skew correction at the > points marked in green (attached). i'm assuming that those diagonal > cross-paths (between each green ring) are... within acceptable > tolerance for intra-skew. > >> If we can do it, the most effective place for intra-pair skew compensation >> is within 15mil of the skew source--right before or after a bend. >> If skew originates in a bend and is resolved by a complementary bend within >> 15mils, >> then we don't need to add anything specific. > > mmmm *grumble, grumble*.... i think there might be space to add them, > around where the green rings are, by moving the diagonal pieces to the > right a bit. > >>>> How far are the differential traces from board edge at present? >>> >>> 0.9mm -> 35 mil. >>> >>> to the nearest vias is 0.2mm -> 0.787mil >> >> How far is the board-edge ground shield trace >> from the edge of the board? > > to the edge of the GND shield trace: 0.46mm -> 18 mil > >> From the closest differential pair trace? > > to the edge of the CK diffpair, 0.93mm -> 36.6 mil > >> How wide is the board-edge ground shield trace? > > pffh :) peanuts. very tight. 13 mil (that's to the vias as well, > which i realise is slightly dodgy). > > >> I'm guessing you meant the closest vias to the differential pair >> traces are 0.2mm = 7.87mil? > > yyep. > >> Are these the ground-to-ground vias for low-impedance connection >> of reference planes? (Low-impedance return path close to signal vias?) > > honestly i haven't been thinking in terms so specific: i just add > them arbitrarily because i heard it was the right thing to do! > learning fast... > > l. > <Untitled.jpg> > _______________________________________________ > 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
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