AM625: DDR CS not being activated as expected

Part Number: AM625

We are using the AM625 with nearly the same DRAM as the reference design:  MT40A1G16TB-062E.  During SPL (tiboot3.bin), we find that when it accesses DRAM, DQS and other signals are generated as expected, but not CS.

CS = yellow, DQS = blue, DDR clock = red.

The CS signal goes only between the AM625 and the DRAM.  Any suggestion about what could be causing it to operate this way?

Thanks,
    Steve