PC-133: SDRAM Certified for use with a 133MHz Bus (7.5nSec) Clock. PC-133: Unlike PC-100, PC-133 is not a formal specification. Primarily "PC-133" means that "CAS" specifications will be in terms of 133MHz clock ticks rather than 100 MHz Clock ticks. PC-100 CAS 2 (20nSec) is roughly equivalent to PC-133 CAS 3 (21.5nSec). It is usually possible to use PC-100 memory at 133MHz provided proper timing is specified. But note that a 100MHz bus running with 2 clock delays is actually a bit slower than a 133MHz bus with 3 clock delays.
It is also worth remembering that "CAS style" specifications are only part of the full DRAM Access cycle. Thus PC-133 CAS-3 memory would be roughly equivalent to 43nSec DRAM.
There are three different CAS specifications -- CAS High Time ("Precharge"), RAS to CAS time, and CAS to valid data.
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Copyright 1994-2002 by Donald Kenney.