The timing looks the same for the 4 Gb part vs. the 1 Gb part used on the tower board. We have successfully made at least three layouts with the K4B1G1646G DDR3 as used on the Tower board. We added impedance controlled and matched length traces for A13 and A14. We modified the value in DDR_CR073 to indicate the two additional rows, but Uboot hangs right when it tries to jump to running from RAM. The new DDR3 part is K4B4G1646B which seems to have the same timing at the old part. I have reviewed various posts and looked at register contents after a successful boot on the same board with the 128 MB part vs a failed boot with the 512 MB part. It seems we must be missing something basic. Any help would be appreciated.