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On Tue, Mar 7, 2017 at 6:46 PM, Hrvoje Lasic lasich@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.