[Arm-netbook] eoma68 rk3288 testing

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Tue, Mar 7, 2017 at 2:07 PM, Hrvoje Lasic <lasich at gmail.com> wrote:
> 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.

 http://linux-sunxi.org/A10_DRAM_Controller_Calibration#Impedance_settings.2C_ODT_and_ZQ_calibration

 apparently ZQ config registers do exactly that

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

 yyyeah not in this case - there's not enough space between the rk3288
and the ddr3 ram ICs to fit 20 0402 resistors, even if i used 4-packs.
i'd need to completely redo the entire DDR3 layout and that's
something that i know will cost about 4 or 5 revisions (and around
$10k in the process).

> 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.

 yeah i will be checking the PMIC output voltages with a scope
tomorrow.  it's fairly certain that it's impedances - going from 290
to 300mhz really should not make a huge difference but it does.

l.