DDR2 Memory Tutorial

Introduction.

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.

Physical Aspect

DDR and DDR2 modules have the same physical size, but DDR modules have 184 contacts, while DDR2 modules have 240. On Figure 1 you can compare the difference between DDR2 and DDR edge contacts.

Figure 1: Differences on edge contact between DDR and DDR2 modules.

Thus there is no way to install a DDR2 module on a DDR socket and vice-versa.

All DDR2 chips use BGA (Ball Grid Array) packaging, while DDR chips almost always use TSOP (Thin Small-Outline Package) packaging. There are DDR chips with BGA packaging on the market (like the ones from Kingmax), but they are not so common. On Figure 2 you can see how a TSOP chip on a DDR module looks like while on Figure 3 you can see how a BGA chip on a DDR2 looks like.

Figure 2: DDR chips almost always use TSOP packaging.

Figure 3: DDR2 chips use BGA packaging.

Resistive Termination

On DDR modules the necessary resistive termination is located on the motherboard, while on DDR2 modules this termination is located inside the memory chips – technique called ODT, On-Die Termination.

This is done in order to make the signal “cleaner”. On Figure 4 you can see the signal that reaches the memory chip. On the left hand side you see the signals on a system that uses motherboard termination (DDR memories) while on the right hand side you see the signals on a system that uses on-die termination (DDR2 memories). Even a layman can easily say that the signals on the right hand side are cleaner and stable than the signals on the left hand side. On the yellow square you can compare the time frame difference – this time frame is the time the memory has to read or write a piece of data. With the use of on-die termination this time frame got wider, allowing higher clocks to be achieved since the memory has more time to read or write a data chunk.

Figure 4: Comparison between motherboard termination and on-die termination.

Latencies

DDR2 memories work with higher latencies than DDR memories. In other words, they delay more clock cycles to deliver a requested data. Does this mean that DDR2 memories are slower than DDR memories? Not necessarily. As we said, they delay more clock cycles, but not necessarily more time.

If we compare a DDR memory to a DDR2 memory running under the same clock, the one with the lower latency will be the fastest. Thus, if you have a DDR400 CL3 memory and a DDR2-400 CL4 memory, your DDR400 will be faster.

Keep in mind that DDR2 memories have an additional parameter called AL (additional latency), which must be added to their nominal latency (CL) in order to get the total latency.

When comparing memories with different speeds, you need to consider the clock in your math.

On a DDR400 CL3 memory, this “3” means that the memory delays three clock cycles to start delivering the requested data. Since this memory runs at 200 MHz, each clock tick measures 5 ns (T= 1/f). Thus its latency if of 15 ns.

Now on a DDR2-533 CL3 AL0 memory, this “3” also means that the memory delays three clock cycles to start delivering the request data, but since this memory runs at 266 MHz, each clock tick measures 3.75 ns, so its latency is of 11.25 ns, making this memory faster to data delivery than our DDR400 CL3 memory. So a DDR2-533 CL4 and AL0 memory has the same latency as a DDR400 CL3. Notice that we are assuming the additional latency as zero, or we would need to take it into account, i.e., a DDR2 CL3 AL1 memory has in reality a latency of four clock cycles.

Some manufacturers announce their memory module latencies thru a series of four number, like “4-4-4-12” or “5-4-4-9” or “3-3-3-8”. The latency we’ve been talking about (CL) is the first number on the sequence. The additional latency (AL) is usually found on the memory module technical specs (usually a PDF file for downloading on the manufacturer website).

In order to make you calculations and comparisons easier, we prepared the following table containing the clock tick duration depending on the memory type. So, just get the number below depending on the memory type you want to compare and multiply it by the latency value in order to know the latency duration in nanoseconds, allowing you to compare latencies of memories with different clock speeds and to know which memory is faster.