Manufacturer's rating of DRAM speed are a maximum rating. For example a chip rated as "PC133" means the chip is rated to a maximum speed of 133MHz. An analogy would be a rope rated as 133lb test could hold a 100lb weight or a 133lb weight. This same principal applies to DDR memory where a PC3200 (400MHz) part is also a PC3000, PC2700, PC2400, PC2100, & PC1600 because it can operate at any speed slower than 400MHz. Memory specifications like DRAM type, cas latency, density, board layers and other factors always have to match the system regardless of the maximum rating of the module.
To better illustrate the point, we have included a chart from Micron that shows the possible uses for 2 different DRAM:
This information is found on Micron's website at: http://www.micron.com/products/category.jsp?path=/DRAM/SDRAM&edID=16464
Micron supports speed grades that are compatible with 66 MHz, 100 MHZ, and 133 MHz data bus speeds. Micron's 64/128/256Mb SDRAMs have two speed grade marks, -75 and -7E. The chart below references these two speed grade marks to its compatible data bus speed.
|Speed Mark||Clock Frequency||Speed Reference||PC Reference|
|-75 or -7E||66 MHz||2-2-2||PC66|
|-75 or -7E||100 MHz||2-2-2||PC100|
|-75 or -7E||133 MHz||3-3-3||PC133|
** CL = CAS Latency READ command to first data out
tRP = Precharge command period
tRCD = ACTIVE to READ or WRITE delay
Please refer to data sheets for more information
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