Confusion in DDR timing parameters

[garimella]

1.In DDR timing, there is a parameter called tRAS, denoting active to pre-charge delay, but tRAS= CL+tRCD, since it is a derived parameter, why is it listed as a critical timing factor as it will depend on worst case CL and RCD actuation.

2.What is the delay between DQS, DM and CLK.?

 

[sharath666]

1) tRAS is not derived.

2) DM is centre aligned with respect to DQS. Clock should not come into this picture.