DDR2 memories are already supported on high-end motherboards. We compiled below a short list with the main differences between DDR2 and DDR memories.

• DDR memories are officially found in 266 MHz, 333 MHz and 400 MHz versions, while DDR2 memories are found in 400 MHz, 533 MHz, 667 MHz and 800 MHz versions. Both types transfer two data per clock cycle. Because of that the listed clocks are nominal clocks, not real ones. To get the real clock divide the nominal clock by two. For example, DDR2-667 memory works in fact at 333 MHz.
• DDR2 memories have a lower power consumption compared to DDR memories.
• DDR memories are fed with 2.5 V while DDR2 memories are fed with 1.8 V.
• On DDR memories the resistive termination necessary for making the memory work is located on the motherboard, while on DDR2 memories this circuit is located inside the memory chip. This is one of the reasons why it is not possible to install DDR2 memories on DDR sockets and vice-versa.
• DDR modules have 184 contacts, while DDR2 modules have 240 contacts.
• On DDR memories the “CAS Latency” (CL) parameter – which is the time the memory delays delivering a requested data –, can be of 2, 2.5 or 3 clock cycles. On DDR2 memories CL can be of 3, 4 or 5 clock cycles.
• On DDR2 memories, depending on the chip, there is an additional latency (AL) of 0, 1, 2, 3, 4 or 5 clock cycles. So in a DDR2 memory with CL4 and AL1 the latency is 5.
• On DDR2 memories the write latency equals to the read latency (CL + AL) minus 1.
• Internally the controller inside DDR memories works preloading two data bits from the storage area (task known as “prefetch”) while the controller inside DDR2 memories works loading four bits in advance.

These are the main differences between DDR and DDR2. We will explore them a little bit more on the following pages. For a more detailed explanation we recommend you to read the following document: http://download.micron.com/pdf/pubs/designline/dl3Q03.pdf.