Xilinx has recently changed their recommendation for DDR3 CKE. ZedBoard Rev C implements a 4.7K-ohm pulldown to GND. The new recommendation is a 40-ohm pull-up to Vtt.
The Micron spec only requires CKE to be low 10ns before RESET#, unlike DDR2 where the term was longer. Since the Zynq controller drives CKE low, that 10ns requirement is met. The termination to VTT could be important for self-refresh users, as CKE will toggle more often than just startup. The 4.7K pulldown may lead to poor SI on this potentially high-frequency signal.
Bryan
