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From Theory to Practice
So, what does all of this mean to you when selecting a memory module?
First, in order to guarantee PC100 compliance, the chips on the module must be specified as having a 6 ns tAC, and a "cycle time" of 8 ns. These parameters can usually be found on the chip manufacturer's web site. You will need the chip code to determine this, but the data is readily available in the data sheets (and sometimes in a table conveniently provided by the manufacturer).
Sometimes you will see modules indicated as being "3-2-2" or "2-2-2". What these numbers represent are the CAS Latency, tRP and tRCD values, respectively, in clock cycles at 100MHz. Note that for any other speed of operation, such as 66MHz or 133MHz, these numbers would change. For example, let's assume that for a given module, tCAC is 25ns, tRP is 20ns and tRCD is 20ns. This would indicate 3-2-2 timings at 100MHz, but what would they be at 133MHz? Since 133MHz is a 7.5 ns clock cycle (tCLK), you would have timings of 4-3-3--which would be invalid for SDRAM since CAS Latency cannot be higher than 3. What does this mean? Basically that you would not be able to operate this module at 133MHz (perhaps you don't care, but it is nice to know).
This brings us to the often hyped CAS Latency figure ("CAS2" or "CAS3"). Many vendors are marketing their "top of the line" PC100 SDRAM as CAS2, and claim this makes it "much more overclockable". While this is generally true, it isn't always the case. Remember that the CAS Latency number is derived from tCAC, and the manufacturer will never give you the tCAC value. If we were going to "stretch" things a bit, we could imagine a CL2 part with a 20ns tCAC and a CL3 part with a 21ns tCAC. At 133MHz (7.5 ns clock cycle), both parts would have a CAS Latency value of 3. What this shows you is that while a CL2 part may be better able to handle a faster system clock speed than a CL3 part, it isn't necessarily so. A "real world" example of this would be the Micron -8C and -8E parts. The -8C part is defined as CL=3 at 100 MHz, while the -8E part is CL2 at 100MHz. However, at 125MHz both parts are defined as CL=3 parts. If you don't plan on running your SDRAM faster than 125 MHz, the -8E part doesn't provide any additional benefit over the -8C part. The message here is, first determine what your requirements are and then select the part that meets (or exceeds) that need--and don't pay extra for marketing hype.
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