On Thu, Aug 17, 2017 at 12:01 AM, Richard Wilbur
<[email protected]> wrote:
>> ahhhh ok. i'm glad you're paying attention :)
>
> I'm trying ;>)
:)
> […]
>> if i just take *out* the ground intermediary traces that would do the
>> trick of bringing the impedance back up, is that right?
>
> Should be a major step in the right direction.
>
>> what would you suggest, here - leave the intermediary GND traces in
>> or take them out.
>
> My suggestion here would be to remove the GND traces between differential
> pairs since we have established that we can't get 15mil clearance from the
> differential pair traces with the GND traces in place. We don't have enough
> room for that.
ok.
> I would also look carefully at the GND traces separating the differential
> pairs from board edge and other circuitry. If we can't put 15mil between the
> differential pair traces and these GND traces, I would remove these GND
> traces as well. If we have to remove the GND traces between differential
> pairs and other circuitry, this will at least have the happy effect of
> providing 15mil spacing between the differential pair and that other
> circuitry.
flood-fill will just end up putting them back - i'd have to set a
copper-to-trace separation @ 15mil as well.
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.
> This is all based on the fact that we are using differential-mode
> transmission for the high-frequency HDMI signals.
>
>> also, i think i "Get It" about the intermediary wiggles. when the
>> transmit end does automatic compensation that results in the signals
>> coming out in such a way that, really, the inter-pair length-matching
>> should be done from the *OPPOSITE* end i.e. from the CONNECTOR.
>
> Maybe I misunderstood the standard because that wasn't my understanding.
> (All I know is second-hand because there are no freely available copies.)
> What I understood was:
> 1. The receiver has the capability to recover up to 5 bit times of
> inter-pair skew,
o arse: *receiver* not transmitter.
> Thus, in order to make an HDMI v1.4 standard-compliant transmitter (which is
> my understanding of what we are trying to do with the EOMA68-A20) we must
> emit from our HDMI connector an HDMI signal which exhibits
> max{T(inter-pair skew)} <= 0.2 * T(pixel) = 588ps
> This inter-pair skew can come from connector, the chip, and the PCB traces
> connecting them. It seems likely that the connector and the chip will likely
> be very minimal sources of inter-pair skew, and thus most, if not all, of the
> transmitter allocation falls to the PCB designer to use (or
> squander--depending on how you view it) in connecting the chip to the HDMI
> cable connector.
>
> At the speed of propagation of signals in our microstrip differential pairs
> this amounts to
> max{length(inter-pair skew)} = v(propagation) * max{T(inter-pair skew)}
> = 150um/ps * 588ps = 88.2mm
> Toradex suggests we limit the inter-pair skew in the traces to 1/4 of that
> value or 0.5 * T(bit) which corresponds to a length of 22mm.
22 mm... okaaay.
> From what I've seen, even without inter-pair skew compensation in the layout
> the inter-pair skew you observed was ~8mm < 22mm.
9. or so. okaaay now i get it.
> If this is indeed how it works then I'll need to rethink my recommendations.
> (I outlined my understanding above.)
nono, my mistake.
>>> Are you talking about moving the differential pairs further
>>> from the edge of the board?
>>
>> yes. but from what you're saying it's not possible anyway.
>
> How far are the differential traces from board edge at present?
0.9mm -> 35 mil.
to the nearest vias is 0.2mm -> 0.787mil
l.
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