[Arm-netbook] eoma68 rk3288 testing

[Hrvoje Lasic]

ok, these are internal resistors that help match impedance automatically.
they are calibrated against reference resistors as they are not very
precise. quite smart. but i still think you need to check impedance of your
lines as i assume this calibration process is good to certain point, if
lines are really bad that it cant help. However, you should try to enable
this, maybe it will help.

Also,before redesign all board, check what impedance should be for all
lines (datasheets), go through simulation software what impedance is now.
Check what was impedance on previous version of PCB that worked (go through
simulation again). So, maybe you can just change thickness of prepag (if
that is possible) that match previous version of PCB (if you have same
design of ddr3/mcu).


On Tue, Mar 7, 2017 at 7:16 PM, Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> ---
> 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.
>
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