[Arm-netbook] Arm Netbook, Saw the update,

[Richard Wilbur]

>> On Jul 4, 2017, at 09:50, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:
>> 
>> On Tue, Jul 4, 2017 at 4:15 PM, Richard Wilbur <richard.wilbur at gmail.com> wrote:
> […]
>> A couple questions that spring immediately to mind:
>> 1.  How continuous are the ground planes under the differential pairs?  (Are there voids in the ground planes under the differential pairs?)
> 
> no.  layers 2 and 5 are solid ground planes.  vias obviously go
> through those , full vias only.  non-GND vias also obviously create
> small interruptions but that's all

Wonderful!  That's music to my ears.  No major obstacles on return current path except the vias.  So when we change signal layers, we'll need adjacent ground plane-to-ground plane vias to provide a nice low-impedance path for the return current in the ground planes.  

>> 2.  The intra-pair length sounds well-matched, my understanding of the inter-pair skew is that the requirement is in terms of the clock speed Δt <= 0.2 Tcharacter.  For 225 MHz clock the application note spoke of 888ps.  So we'll be interested in determining the maximum clock rate and speed of propagation.
> 
> 3e8 m/sec * 888e-12 = 0.2664 metres.... which is 266.4 mm... way
> beyond anything which would indicate some kind of problem if the board
> is 78mm long and the skew is only 2mm.

You're on the right track and that is a good calculation for propagation in free space (vacuum).  It turns out with relative permittivity of FR-4 fiberglass substrate our signal actually travels significantly slower which tightens our design parameters a little.

>> I'll also have some questions about dimensions and geometry.
> 
> ack.


Not too awful.  Just a few questions about how you are routing to control the impedance.
1.  Are you using a 2-D field solver to determine the impedance and adjust design parameters accordingly(trace width, intra-pair spacing, inter-pair spacing, etc.)?
2.  Are you working with your board fabricator to determine the design parameters mentioned above b