[Arm-netbook] KiCAD based A10 break out / prototyping PCB finished
joem at martindale-electric.co.uk
Wed Feb 6 16:58:43 GMT 2013
On Wed, 2013-02-06 at 07:47 -0600, Bari Ari wrote:
> On 02/06/2013 07:02 AM, jm wrote:
> > I agree Ajith there is a lot of R&D and knowledge out there
> > about reflections and PCB design. Please do continue to tell me I'm
> > wrong and why - because all these links enriches us all. My only defence
> > is that it doesn't agree with my experiences. Its a chance to check up
> > on all the theory and if I'm wrong, I'll admit to it, explain what went
> > wrong and redo the PCBs. I have budget for several iterations, so I'm
> > not desperate to get it right first time, second time etc.
> There are critical timing issues with DDR3. You have multiple lines of
> data and control signals. The data lines have to be stable before they
> are latched or the stored data won't be correct. Its the same for reads,
> the CPU DDR3 controller needs the data lines to be setup and stable
> before they are latched. So data needs to arrive slightly first, then
> control and clocks. There are lots of lines and they need to be matched
> in length as well as impedance or the control states or data will be
> corrupt. There are also other issues here that I'm not going into with
> high speed PCB design like reflections, dynamic impedance, skin effect,
> via design, series resistance, etc etc.
> Is it possible to do this with limited PCB tools like Kicad. Yes it is
> but it depends on some luck and lots of wasted manual time spent pushing
> and shoving things around. Sometimes people get lucky and their PCB
> layout works. There is a reason why experienced high speed designers use
> tools like Allegro or Altium for PCB layout since they can get solid
> boards designed in a fraction of the time.
> A software analogy might be: Can you use punch-cards or BASIC to write
> an application for the A10? Sure but do you really want to?
I don't disagree with any of it and I have the skills to engineer it
properly taking all those issues into account even if manually done
However, I find it difficult to take on board some of the things
said in the A10 manuals because it does agree with my training,
and hopefully it can be checked with break out boards before engineering
something deep and untestable.
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