[Arm-netbook] eoma68 rk3288 testing

[Hrvoje Lasic]

impedance is function of width of line (with some adjustment of copper
thickness) plus thickness of prepag directly below surface where your lines
are. So, you need to compare what is on your last pcb and what is on your
new pcb. best software to calculate this value is polaris and you can get
some days for free if you ask for code but there are free alternatives out
there like Saturn that we use. Beside that there are differential lines
that you also need to match impedance. I am not sure but I doubt that via
registers you can fix this problem.

also, in some cases you can put resistors in series of lines where you
think you have problem.

basically, you need to find out what impedance need to be and where are you
now in order to fix this, of course if this is really problem.

On Tue, Mar 7, 2017 at 2:54 PM, Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> http://rhombus-tech.net/rock_chips/rk3288/news/
>
> i now have 2 assembled PCBs with the exact same "fault" - which after
> talking to paulk and akaka on #linux-rockchip i *may* have a lead on
> how to get the DDR3 RAM up and running properly.  we believe it's down
> to the impedance control register (ZQCR) which is being hard-coded in
> u-boot-rockchip rather than being allowed to "train".  those values
> are suited for 6-layer 1.6mm PCBs - the impedance of tracks when you
> have an 8-layer 1.2mm PCB are *half* what they are for 6-layer 1.6mm
> PCBs.
>
> l.
>
> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
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