What does DDR2 mean?

DDR2
DDR2 (double data rate 2) SDRAM is a new generation of memory technology standard developed by JEDEC (Joint Committee for electronic equipment engineering). The biggest difference between it and the previous generation of DDR memory technology standards is that, although the same basic method of data transmission is adopted at the same time as the clock increase/decrease delay, DDR2 memory has twice the pre-read capability of the previous generation of DDR memory (I .e: 4-bit data read prefetch ). In other words, each DDR2 memory clock can read/write data at a speed of 4 times that of the external bus, and can run at a speed of 4 times that of the internal control bus.

In addition, due to DDR2 standard, all DDR2 memory adopts fbga encapsulation form, and different from the current widely used TSOP/TSOP-II encapsulation form, the fbga package provides better electrical and thermal properties, providing a solid foundation for the stable operation of DDR2 memory and future frequency development. Recalling the development history of DDR, ddr200, from the first generation of applications to personal computers, has gone through ddr266 and ddr333 to today's dual-channel ddr400 technology, the development of the first generation of DDR has reached the technical limit, and it is difficult to improve the memory speed through conventional methods. With the development of Intel's latest processor technology, the front-end bus has higher and higher requirements on memory bandwidth, and DDR2 memory with higher and more stable operating frequency will be the trend of the times.

Difference between DDR2 and DDR:

1. latency:

From the table above, we can see that the actual operation frequency of DDR2 is twice that of DDR at the same core frequency. This is because DDR2 memory has twice the 4-bit pre-read capability of the standard DDR memory. In other words, although DDR2 and DDR both adopt the basic method of data transmission at the same time as the clock increase delay and decrease delay, but DDR2 has twice the ability of DDR to pre-read system command data. That is to say, in the same 200 MHz operating frequency, the actual DDR frequency is 400 MHz, while DDR2 can reach MHz.

In this case, another problem occurs: In the same working frequency of DDR and DDR2 memory, the latter's memory latency is slower than the former. For example, DDR 200 has the same latency as the DDR2-400, while the latter has twice the bandwidth. In fact, DDR2-400 and DDR 400 have the same bandwidth, they are 3.2 Gb/s, but ddr400 core operating frequency is 200 MHz, while DDR2-400 core operating frequency is 100 MHz, that is to say, the DDR2-400 latency is higher than ddr400.

2. encapsulation and heating:

The biggest breakthrough of DDR2 memory technology is not that users think that it is twice the transmission capability of DDR, but that it uses lower heat and lower power consumption, DDR2 can improve the frequency faster, breaking the MHz limit of the standard DDR.

The DDR memory is usually packaged in the form of TSOP chip, which can work well at MHz. When the frequency is higher, its long pin will produce high impedance and parasitic capacitance, this will affect its stability and Frequency improvement. This is why the core frequency of DDR is difficult to break through MHz. DDR2 memory adopts fbga encapsulation. Unlike the widely used TSOP encapsulation, fbga encapsulation provides better electrical performance and thermal dissipation, ensuring the stable operation of DDR2 memory and the development of future frequencies.

DDR2 memory uses a voltage of 1. 8 V. Compared with the DDR Standard of 2.5 V, DDR2 memory reduces a lot, thus providing obvious smaller power consumption and smaller calorific value. This change is of great significance.

New technologies used by DDR2:

In addition to the differences mentioned above, DDR2 also introduces three new technologies: OCD, ODT, and post CAS.

OCD (off-chip driver): the so-called offline driver adjustment. ddr ii can improve signal integrity through OCD. Ddr ii makes the two voltages equal by adjusting the pull-up/pull-down resistance values. The use of OCD improves signal integrity by reducing the skew of the DQ-DQS; by controlling the voltage to improve signal quality.

ODT: ODT is the ending resistor with a built-in core. We know that the motherboard that uses ddr sdram needs a large amount of ending resistance to prevent the data line terminal from reflecting the signal. It greatly increases the manufacturing cost of the motherboard. In fact, different memory modules have different requirements for the ending circuit. The size of the ending resistance determines the signal ratio and reflectivity of the data line, if the ending resistance is small, the data line signal reflection is low but the signal-to-noise ratio is low. If the ending resistance is high, the data line signal-to-noise ratio is high, but the signal reflection will also increase. Therefore, the ending resistance on the motherboard does not match the memory module very well, but also affects the signal quality to a certain extent. DDR2 can build an appropriate ending resistance based on its own characteristics, so as to ensure the best signal waveform. DDR2 can not only reduce the cost of the motherboard, but also get the best signal quality, which is incomparable to DDR.

Post CAS: it is set to improve the efficiency of ddr ii memory utilization. In the post CAS operation, the CAS signal (read/write/command) can be inserted into a clock cycle after the RAS signal. The CAS command can be effective after the additional latency (additive latency. The original trcd (RAS to CAS and latency) was replaced by Al (additive latency). AL can be set in 0, 1, 2, 3, 4. Since CAS signals are placed behind the RAS signal in a clock cycle, the Act and CAS signals will never conflict with each other.

In general, DDR2 adopts many new technologies and improves the many shortcomings of DDR. Although it has many shortcomings such as high cost and slow latency, it is believed that with the continuous improvement and improvement of the technology, these problems will eventually be solved.

What does DDR2 mean?