What DDR2 SDRAM means
DDR2 SDRAM stands for double data rate synchronous dynamic RAM. The 2 is the DDR2 stands for the second generation of the dual data rate RAM. They are the successors of the DDR1 or simply the DDR. DDR2 RAMsare capable of operating the external bust almost twice as fast as the DDR1 RAM.
We had explained the different types of computer random access memory (RAM) earlier in an article. The word dynamic means that the RAM requires regular refreshing since the data can be lost since electrons leak from time to time. There is the SRAM that does not require refreshing but they are too expensive to adopt for commercial purposes. Synchronous means that the RAM is synchronized with the CPU.
The double data rate means that the data is transferred during the rising and the falling edge of the clock cycle. Technically, it is twice as fast as the single data rate RAMs. Single data rate RAMs are no longer used today. Double data rate RAM has simply been improving with each generation. As of 2021, DDR5 was almost in the market.
History of DDR2 SDRAM
The first DDR2 was commercially available in 2001, a Samsung brand. It is a move that comes with its rewards. Samsung received a technical recognition award from JEDEC. The company efforts were rewarded for developing as well as standardizing DDR2.
The initial clock speeds of the first DDR2 RAMs were 266Mhz and 200MHz. As an analyst, you could have been in for a rude shock since the two versions of RAMs give worse performance than their predecessor. The newly introduced RAMs had a surprisingly higher latency rate and so the access time was longer than that of DDR1. Though the DDR1 RAM modules managed to offer desirable performance, the JEDEC did not allow their standardization.
It was only in 2004 that DDR2 started to compete with DDR1 when the manufacturers managed to reduce the latency.
Double channel /single channel
Motherboards are capable of handling sing channel, two-channel, or even four channels. To differentiate between one channel and double channel RAM slots in the motherboards manufacturers use different colors. DDR memory modules have to be inserted in pairs.
Since motherboards are designed to use only of the many generations of the DDR RAMs ( DDR1, DDR2, DDR3, or DDR4), the RAM modules ( or RAM sticks) and motherboard slots have notches placed differently. This is to ensure you do not use the DRR1 stick on a motherboard that is designed to use DDR2 or DDR3 RAMs.
Figure 1:DDR1, DDR2, DDR3, and DDR4 have notches located differently.
So why can you tell the difference between DDR2 and the other types of memory RAM? The notch on the DDR2 is slightly offset from the middle and the notch is thinner than that in DDR1.
The typical clock speed for DDR2 SDRAM is 100 MHz, 133 MHz, 166 MHz, 200 MHz, and 266MHz. Just as it is with NVMe ( non-volatile memories), DDR2 comes in various sizes, 512 MB, 1Gb, 2Gb, 4Gb, and 8Gb ( this is the highest capacity).
It is in your interest to note that DDR2 RAMs may be phased out soon. Therefore, if you are going for RAM make sure you know its generation to see if it is compatible with your motherboard. Otherwise, if you buy DDR2 RAM for a DRR3 motherboard you will have to return it and reorder a correct RAM stick for your motherboard.
DDR2 modules ( or RAM sticks) are not backward compatible with the DIMMs designed for DDR1. Other than a notch location, DDR2 has a different number of pins than DDR1. While the latter has only 184 pins, the former was designed with 240 pins! You should also know that the DDR2 DIMMs are available in versions, the high speed, and lower speed. However, this should not scare you since you can mix them; the memory controller will take care of things for you.
And how is DDR2 superior to DDR1 RAM? For starters, you will realize that DDR2 is designed to allow higher bus speeds. Additionally, DDR2 requires less power than DDR1 since it runs its internal clock at half the speed of the data bus. When the ability to transfer data on rising and falling edges of the clock cycle is combined with running the internal clock at half the speed, per clock cycle you get four data transfers. DDR2 SDRAM has reduced less latency than DDR1. Lower latency is achieved by the fact that the DDR2 has its internal clock running at half the speed of the external clock rate but at the rate, therefore, being able to provide the same bandwidth. In other words, DDR2 operates at twice the data bus clock rate of its predecessor.
Do you remember with had said earlier that DDR2 has 220 pins while DDR1? Well, this means one thing, DDR2 must have an increased prefetch length. While the DDR1 had a single word for every two bits, DDR2 has four bits for every single word.
Increased prefetch length
When accessing the DDR2, you either write or read from a four-bit-deep prefetch queue. An increased prefetch length is the attribute that made the DDR2 have a doubled rate of data transfer over the data bus. The design was subtly developed to ensure that the transfer rate was increased without increasing the power consumption.
We had said earlier that the first DDR2 RAM had very higher latency, a mistake Samsung worked hard to rectify. DRR1 had a read latency of up to three bus cycles whereas DDR2 had up read latency up to nine cycles. To overcome this, manufacturers made sure that the DDR2 was operated at twice the data rate to achieve lower latency; at least as low as those of DDR1.
The increased bandwidth that can with the DDR2 RAMs meant they had to be assembled on BGA instead of the TSSOP package. In addition to being expensive, your DDR2 BGA package is understandably difficult to assemble.
Lower power requirement
DDR2 RAM can astoundingly save power because of the improved manufacturing process. The dies are significantly shrunk causing the operating voltage of DDR2 to reduce to 1.8 v. DDR1 operating voltage was 2.5 V. moreover, the reduced power consumption attributed to this memory is also due to lower clock frequency when the RAM is used in operations that do not require high data rates.
Standards established by the JEDEC indicate that the highest allowed voltage for the DDR2 is 1.9 v. This is the absolute maximum to go for when stability becomes an issue. However, the DDR2 RAM is designed to withstand up to 2.3 v before it is damaged permanently. You should not set your voltage to this level since the RAM may not function correctly.
Standard and modules
There are about 6 standards for the DDR2 RAMs which are DDR2-400, DDR2-533, DDR2-667, DDR2- 800, and DDR2-1066. The standards are meant to describe the data transfer rate on each raw RAM stick.
On the other, some manufacturers have an ingrained tendency to label their DDR2 RAM using the PC2-xxxx instead. Unlike the other naming system, the PC-xxx method denotes the bandwidth of your RAM. The last two digits are usually truncated.
Error-correcting code (ECC)
Bandwidth is a product of the transfer rate per second and eight (8). We multiply by eight since the DDR2 RAM is used as a 64-bits data bus and each bit is 8 bytes. There are RAMs designed with an extra data byte lane for error-correcting code (ECC). As the name suggests, the error-correcting code is used for correcting and detecting minors and major errors respectively. This increases the reliability of the RAM several folds.
And how can you tell your RAM has ECC or not? In addition to the names given above, PC-xxxx, manufacturers also add ECC or P at the end of the name. for instance, you can have PC2-8500 ECC or PC-3200P. ECC RAM is designed for server use and the amount of data you need to transfer does not require correcting since it is significantly small compared to that transferred in servers.
You will also be astounded to realize that the DDR2 RAM also used to be buffered are still are. Buffering was meant to increase the integrity of the signal. This came at a cost. There was an increased latency because of the extra clock signal. When you find a buffered (also referred to as registered) RAM you will know since it has an additional R in its name. for instance, a registered PC-6400 RAM will be labeled PC-6400R. If it is buffered and has an error-correcting code, it will be designated as PC-6400R ECC. RAM buffer appears as shown below.
Figure 2: A memory buffer
You should know there is another category of DDR2 SDRAM that is referred to as fully buffered memory modules. Instead of an R, manufacturers use F or FB to denote them. Do not worry since they have notches placed in a different location from the normal DDR2. You would not and cannot use them in your motherboard because of the differently positioned notch.