DDR-2 in a bit of detail
While it has been covered on a few other sites, we really haven't touched on it here at TweakTown in terms of the improvements that DDR-2 has over DDR.
DDR used a simple technique - take the existing SDRAM memory and use both the rising and falling edges of the clock cycle to transmit data rather than just the falling of SDRAM. This allowed the memory buffer to send 2 bits of data per clock rather than 1.
DDR-2 uses a 4 bit buffer, so rather than sending 2 bits from the memory cell to the memory buffer, it sends 4. This is why DDR-2 manages to scale so far. To get 400MHz out of DDR, you need the memory cell to be running at 200MHz. To get 400MHz out of DDR-2, the memory cell only runs at 100MHz. This is why we have DDR-2 800 already. We are using the same 200MHz clocked cells, only for DDR-2 this equates to a rating of 800MHz.
DDR-2 was setup for speed from the beginning, in order to ensure a cleaner data signal; a new feature was added to DDR-2, called On-Die Termination. In DDR, the terminating resistors for signal rebound and noise dissipation were put on the board itself next to the DDR slots. This can and did result in some boards having memory signal issues due to cross talk - the longer the trace path has to go, the more it is susceptible to noise (errors).
DDR-2 modules have a series of resistors built directly into the memory chip that acts as the terminating system. This means there is very little chance of external noise interfering with the memory signal as there are no traces for the termination system external to the chips.
When you look at the actual package of the DDR and DDR-2 modules, they almost look identical. This is where things end; DDR uses a 168-pin interface to connect the memory to the board and DDR-2 uses 240 pins. Electrically these modules are also totally different - DDR uses a 2.5v signal and DDR-2 drops this to 1.8v.
DDR-2 is here to stay with both AMD and Intel ratifying DDR2-800 standards for their latest processors and chipsets, with a total of 6.4GB from a single channel DDR2-800 module, that doubles to 12GB with Dual Channel, its clear to see DDR-2 ahs the advantages. We'll see DDR-3 soon but that is still some time away from being released into the market.