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

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Aug 17 06:22:40 BST 2017

On Thu, Aug 17, 2017 at 12:01 AM, Richard Wilbur
<richard.wilbur at gmail.com> 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.


> 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


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