Scary, I just answered this on another thread today.
A little copy and paste... and...
The different timings are...
tCAS = Time for the Column Access Strobe
tRCD = Time RAS to CAS Delay
tRP = Time RAS Precharge
tRC = Time for a cycle, random read or write; It equals tRAS + tRP
IE. 2:2:2:5 = tCAS : tRDC : tRP : tRC
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