RAM

Speed & Timings

DDR chips are classified as DDRX-Y, where X is the DDR generation and Y the maximum clock speed. For example, a DDR3-1600 is a DDR3 chip running at 1600MHz.

Entire memory modules are classified as PCX-Y, where X is the DDR generation and Y the maximum transfer rate in MB/s.

Memory modules may have a different theoretical transfer rate than the individual DDR chips that makes up the module because these chips tend to run below the maximum clock speed.

Memory timings are written as: CL-tRCD-tRP-tRAS-CMD:

CL	The CL number represents the CAS latency (CL, or Access Time). This value denotes how many clock cycles are required for a memory module to return data. A CL7 will require 7 clock cycles for data to be available.
tRCD	The tRCD number is the RAS to CAS delay. The time needed to activate the line (RAS, Row Address Strobe) and column (CAS, Column Address Strobe) of the data.
tRP	The tRP number is the RAS Precharge delay. It is the time required between disabling and enabling another data line.
tRAS	The tRAS number is the Active to Precharge delay. It is the amount of time required before the next access can be initiated.
CMD	The CMD number is the command rate. It is the time required between activation of the module to when the first command can be sent.

See Also

• https://www.hardwaresecrets.com/understanding-ram-timings/