That’s not the only difference between DDR2 and DDR. Now we should recall such trivial things like signal termination. You should know about that if you’re using a SCSI controller and hard disk drives. In brief, the high-frequency signal reflects from the end of the signal line, masking the useful signal with the reflection noise. In order to prevent this, a bunch of resistances is hung on the end of the line, to muffle the signal completely.
These termination resistors are on the mainboard for DDR memory: you may notice numerous tiny resistors and capacitors in the neighborhood of DIMM slots. Of course, the necessity to wire all this stuff onboard doesn’t make the mainboard cheaper, or easier to manufacture. With DDR2, such devices are placed directly in the memory, thus making it unnecessary to place all those electronics onboard. Of course, this puts stricter demands on the range of the nominal characteristics of a module since modules from different manufacturers must work together right, but all involved are going to win from this solution in the long run.
There’s yet another aspect where DDR2 is preferable over DDR – heat generation. DDR modules of ordinary capacities (256MB – 512MB) are not very hot at work. But the fact is that the module becomes hotter when its capacity grows. For example, installing 4GB of RAM into the slots, we can notice that the memory may dissipate 35-40W under peak loads and that’s not little. Yes, such memory amounts are rare today, but tomorrow? Thus, it is necessary to solve this problem beforehand, writing premises for reducing heat dissipation into the new memory standard. Moreover, the operational frequency (heat is proportional to the clock rate, all other factors being equal) of the memory will significantly grow up.
Well, DDR2 has something it can be boastful of. The DDR2 core will work at 1.8v – compare to 2.5-2.6v of the current DDR (higher voltage means more heat, too). Thus, DDR2 should produce less heat. They estimate a reduction by 30% and practice will prove that. We should notice the fact that a lot of today’s DDR works at 1.8v, which it converts from the input 2.5v voltage. However, such conversions heat the module up, too, so without them the heat dissipation will decrease.
Another innovation employed in DDR2 is called Additive Latency. To understand its point we should know that in real circumstances it is not sometimes possible to transfer data even at the moments when this is formally allowable, because the memory control bus has a limited number of states. So sometimes it’s not possible to send the command to initialize the next memory bank simultaneously with the command for reading the earlier-initialized bank. Just because these two commands require that signals of two opposite levels are sent at once by the same bus. As a result, there’s a bubble in the data stream from the module with no useful data, which the memory couldn’t provide due to this organizational conflict.