DDR2 (Double Data Rate 2) SDRAM is a new generation memory technology standard developed by JEDEC (Joint Electronic Equipment Engineering Committee). The biggest difference between it and the previous generation DDR memory technology standard is that although the same uses the rising clock/ The basic method of data transmission while falling delay, but DDR2 memory has twice the pre-read capacity of the previous generation DDR memory (ie: 4bit data read pre-fetch). In other words, DDR2 memory can read/write data at 4 times the speed of the external bus per clock, and can run at 4 times the speed of the internal control bus.
1. Basic principles:
Compared with DDR, the main improvement of DDR2 is that it can provide twice the bandwidth of DDR memory when the speed of the memory module is the same. This is mainly achieved through the efficient use of two DRAM cores on each device. In contrast, DDR memory can only use one DRAM core on each device. Technically speaking, there is still only one DRAM core on DDR2 memory, but it can be accessed in parallel, processing 4 data instead of two in each access.
Combined with the data buffer running at double speed, DDR2 memory can process up to 4 bits of data in each clock cycle, which is twice as high as the 2 bit data that can be processed by traditional DDR memory. Another improvement of DDR2 memory is that it uses FBGA packaging instead of the traditional TSOP method.
However, although DDR2 memory uses the same DRAM core speed as DDR, we still have to use a new motherboard to match DDR2 memory because the physical specifications of DDR2 are incompatible with DDR. First, the interface is different. DDR2 has 240 pins, while DDR memory has 184 pins. Second, the VDIMM voltage of DDR2 memory is 1.8V, which is also different from the 2.5V of DDR memory.
In addition, because the DDR2 standard stipulates that all DDR2 memories are packaged in FBGA, unlike the currently widely used TSOP/TSOP-II package, the FBGA package can provide better electrical performance and heat dissipation, which is a stable DDR2 memory. Work and the development of future frequencies provide a solid foundation. Recalling the development of DDR, from the first generation of DDR200 applied to personal computers through DDR266, DDR333 to today’s dual-channel DDR400 technology, the development of the first generation of DDR has reached the limit of technology, and it has been difficult to improve memory through conventional methods. With the development of Intel’s latest processor technology, the front-side bus requires higher and higher memory bandwidth, and DDR2 memory with higher and more stable operating frequencies will be the general trend.
Before understanding many new technologies of DDR2 memory, let us first look at a set of data comparing DDR and DDR2 technologies.
1) Delay problem:
As can be seen from the above table, under the same core frequency, the actual operating frequency of DDR2 is twice that of DDR. This is due to the fact that DDR2 memory has twice the 4BIT pre-read capability of standard DDR memory. In other words, although DDR2, like DDR, uses the basic method of data transmission at the same time as the clock rise delay and fall delay, DDR2 has twice the ability of DDR to pre-read system command data. In other words, under the same operating frequency of 100MHz, the actual frequency of DDR is 200MHz, while DDR2 can reach 400MHz.
In this way, another problem arises: in DDR and DDR2 memory with the same operating frequency, the memory latency of the latter is slower than the former. For example, DDR 200 and DDR2-400 have the same delay, while the latter has twice the bandwidth. In fact, DDR2-400 and DDR 400 have the same bandwidth, they are both 3.2GB/s, but the core operating frequency of DDR400 is 200MHz, and the core operating frequency of DDR2-400 is 100MHz, which means the delay of DDR2-400 It is higher than DDR400.
2) Encapsulation and heat generation:
The biggest breakthrough of DDR2 memory technology is actually not that users think twice the transmission capacity of DDR, but with lower heat generation and lower power consumption, DDR2 can achieve faster frequency increases and breakthroughs. The 400MHZ limit of standard DDR.
DDR memory is usually packaged in TSOP chip. This package can work well at 200MHz. When the frequency is higher, its long pins will generate high impedance and parasitic capacitance, which will affect its performance. The difficulty of stability and frequency increase. This is why it is difficult for the core frequency of DDR to break through 275MHZ. And DDR2 memory adopts FBGA package form. Different from the currently widely used TSOP package, the FBGA package provides better electrical performance and heat dissipation, which provides a good guarantee for the stable operation of DDR2 memory and the development of future frequencies.
DDR2 memory uses 1.8V voltage, which is much lower than the DDR standard 2.5V, thus providing significantly smaller power consumption and less heat. This change is significant.
2. New technologies adopted by DDR2:
In addition to the differences mentioned above, DDR2 also introduces three new technologies, they are OCD, ODT and Post CAS.
OCD (Off-Chip Driver): This is the so-called offline drive adjustment. DDR II can improve signal integrity through OCD. DDR II adjusts the pull-up/pull-down resistance value to make the two voltages equal. Use OCD to improve signal integrity by reducing the tilt of DQ-DQS; improve signal quality by controlling voltage.
ODT: ODT is the termination resistor of the built-in core. We know that a large number of terminating resistors are needed on the motherboard using DDR SDRAM to prevent the signal from being reflected at the data line terminal. It greatly increases the manufacturing cost of the motherboard. In fact, different memory modules have different requirements for the termination circuit. The size of the termination resistor determines the signal ratio and reflectivity of the data line. If the termination resistance is small, the data line signal reflection is low but the signal-to-noise ratio is also low; If the termination resistance is high, the signal-to-noise ratio of the data line will be high, but the signal reflection will also increase. Therefore, the termination resistance on the motherboard cannot match the memory module very well, and it will affect the signal quality to a certain extent. DDR2 can build in suitable termination resistors according to its own characteristics, so as to ensure the best signal waveform. Using DDR2 can not only reduce the cost of the motherboard, but also get the best signal quality, which is unmatched by DDR.
Post CAS: It is set to improve the utilization efficiency of DDR II memory. In Post CAS operation, the CAS signal (read/write/command) can be inserted one clock cycle after the RAS signal, and the CAS command can remain valid after the additional delay (Additive Latency). The original tRCD (RAS to CAS and delay) is replaced by AL (Additive Latency), which can be set in 0, 1, 2, 3, 4. Since the CAS signal is placed one clock cycle after the RAS signal, the ACT and CAS signals will never collide.
In general, DDR2 uses many new technologies to improve many of DDR's shortcomings. Although it currently has many shortcomings in terms of high cost and slow latency, it is believed that with the continuous improvement and improvement of technology, these problems will eventually be resolved. .
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