On Tue, Aug 1, 2017 at 8:25 AM, mike.valk@gmail.com mike.valk@gmail.com wrote:
2017-08-01 16:11 GMT+02:00 Luke Kenneth Casson Leighton lkcl@lkcl.net:
*sigh* unfortunately symmetry is not completely achievable with the drastically-reduced amount of space available. the HDMI signals come out right at the bottom of the board.
That might not be to problematic. I've search to the net for talk about running tracks on top of each other. It keeps hunting me. And found a knowledable awnser.
Thanks for the interesting read. It is an intriguing geometry and looks like it uses vertical space more than horizontal. I suppose we could make use of a 2-D EM field solver to figure out design parameters such as trace width, which layers to use (how much dielectric thickness between traces), and horizontal offset from other traces.
On of the things mentioned is that the differential signals might not be quite aligned to begin with. So achieving symmetry might look nice on but can only give limited help in minimizing emission and pickup.
I guess to get better than that we would have to characterize the differential signal sources and, if they have a repeatable output skew, then design the traces with that initial skew from the source. I have a suspicion that the important goals for this project include: 1. operational HDMI v1.4 interface supporting all operating modes of which the A20 chip is capable (clock up to 340MHz, data up to 3.4GHz) 2. EMI radiation below regulatory limits for all concerned agencies: FCC, CSA, EU, etc. 3. EMI sensitivity small enough to avoid disrupting proper operation of all systems of the EOMA68-A20.
Hopefully the drivers on the chip are aligned well enough to fit into the skew budget for HDMI operation and we are able to execute a PCB trace geometry to support all three goals. (I'm willing to bet on it!)