[Arm-netbook] OLIMEX A10 board

[lkcl luke]

On Sat, May 26, 2012 at 11:52 AM, Tsvetan Usunov - OLIMEX Ltd
<usunov at olimex.com> wrote:
>>  the DDR3 RAM circuit, if you do not use PADS, and do not keep those
>> DDR3 connections EXACTLY the same, you will be in for a bit of a shock
>> when it comes to doing the development of the A10 board.
>>
>
> why? let's not act like chinese factory with copy and paste designs :)
>
>>
>>  wits-tech's PADS layout has done the hard part.  if you deviate from
>> that you will need to spend several man-months replicating that costly
>> work.
>>
>
> this is what I read on their PDF schematic:
>
> DQ0-7、DQM0、DQS0 Length matching 100mil
> DQ8-15、DQM1、DQS1 Length matching 100mil
> DQ16-23、DQM2、DQS2 Length matching 100mil
> DQ24-31、DQM3、DQS3 Length matching 100mil
> DA、CONTROL、CK Length matching 300mil
> DQSn、DQSn# Differential pairs Z0= 100 ohm,Length matching 10mil
> CK、CK# Differential pairs Z0= 100 ohm,Length matching 10mil
>
> and this is pretty much same as on the other DDR3 requirements I've seen for
> other processors like AM335X for instance
> seems to me not hard to accomplish simple rules: keep length same and
> impedance as required and there should be no issues with proper ground
> planes.

 you need to read up on application notes about DDR3... or you need to
save yourself some time and effort and copy, verbatim, what wits-tech
have already done.

 there are several rules to follow that i found in various application
notes.  one of them is to avoid as many via holes as you can (keep
them to an absolute minimum).  another is to use "snakes" that turn at
very specific angles, so that the tracks can be made to be exact
matching lengths by the time they arrive.  if you use right-angle
turns then the signal will *not* go round the corner, it will keep
going in a straight line and will cause R.F. interference with other
tracks and also increase EM radiation.

 l.