DDR vs DDR2 performance question

Okay so we now have three families of DDR SDRAM memory out there DDR, DDR2, and DDR3. If you look a little more closely at the datasheets between them you will find that typically the CAS latency remains the same between them for a given clock frequency. So for example, DDR SDRAM at 200 MHz typically has CL=3. Well if you look at the datasheet for DDR2 SDRAM, you will also find that at 200MHz, CL=3. So you don't lose anything in terms of frequency vs. CL scaling.

One thing to make note of is that DDR2 has a maximum tCK of 8.0ns which results in a minimum clock frequency of 125MHz. You must plan on running your DDR2 memory at at least this frequency or something close to it. You can run it slower if you know what you're doing.

So then to your real question. If you run the DDR2 memory at say 166MHz, can you get a CAS latency of 2.5 like you could with DDR1. Well DDR2 does not support half CAS latencies so the answer is no. Could you get a CAS latency of 2? Probably not. and it's not supported. You'd probably have to live with a CAS latency of 3.

What you do get from migration from DDR to DDR2 is lower power and ODT.

Jake