[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 6:46 PM, Hrvoje Lasic <lasich at gmail.com> wrote:
> 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.

 i'll find out tomorrow soon enough.  current settings are hard-coded
to 40ohm driving and 155ohm termination (in the processor dram) which
seems very odd.

> 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).

 not a chance.  increasing the line thickness to 4mil increases the
impedance by something like 4%.  there's no way i can use anything
other than 1.2mm boards.  there's ABSOLUTELY no way i can go to 6
layer.  and the dielectric constant for 6mil board separation would
have to be reduced to something insane like 1.0 in order to claw back
the reduction in separation.

 this *has* to have been taken into account in the design of DDR3
drivers / receivers, that there would be circumstances where the
impedance is off like this.

l.