SDRAM is a relatively long-term thing, but when we talk about it, it will definitely not be confused with DDR. SDRAM as we usually understand it is actually SDR SDRAM, which is the first generation of SDRAM, while DDR1 is the second generation, even now. The DDR4 used is actually the fifth-generation SDRAM, which needs to be clarified here. To show the difference, SDR is used in subsequent articles to refer to SDR SDRAM, and DDR is to refer to DDR SDRAM.
As many people have replied, their essential difference is the difference in periodic operation (also called clock sampling), which leads to a big difference in the subsequent design. SDR is a "single data transfer mode". The characteristic of this memory is to perform an operation (read or write) on the rising edge of a waveform in a memory clock cycle, while DDR refers to some new designs and technologies. In a memory clock cycle, an operation is performed on the rising edge of the waveform, and an operation is also performed on the falling edge of the square wave, which is equivalent to that in one clock cycle, DDR can complete the task that can be completed in two cycles of SDR. Therefore, in theory, the performance of DDR memory with the same speed is more than twice that of SDR memory, which can be simply understood as 100MHZ DDR=200MHZ SDR.
As for how to consider the length of SDR in design, I think this may be the most interesting issue for everyone. Although the application of SDR is not much, people still often ask us. The following is a better one. The answer to the article is also a reference for everyone.
The netizen of Ergaozi said: "Although they are all called synchronous dynamic random access memory, they are very different in technology. Sdram belongs to the first generation ram, and ddr-sdram belongs to the second generation ram. It uses double data rate and pre-access technology. , The transmission rate is more than twice that of the first generation. In the layout, sdram does not even need to be equal in length, except for high performance requirements."
A netizen from Shanshui Jiangnan said: "Sdram is synchronized with a common clock. Data and clock signals do not have to be equal in length, but there is a longest requirement, so the wiring should be as short as possible."
There are some other netizens who have similar views. We agree with this statement. Normally, if the SDR frequency is below 100MHz, the equal length range can be larger. Relatively speaking, you don’t need to deliberately control it, and if the frequency exceeds Above 100MHz, special attention should be paid to PCB design. The isometric length can be calculated through an accurate timing simulation. Our rule of thumb is to control the length of all signals as much as possible. It is best if the length is not more than 3 inches under controllable conditions. . This is mentioned in Mr. Gao's previous article on Timing Design, so I won’t explain it here.
Okay, now officially back to our DDR era.
The transmission speed of the memory can be rapidly increased. In addition to the advancement of the chip manufacturing process, the key technology is double data rate and pre-access. In fact, the core frequency of the memory is basically the same, between 100MHz and 200MHz. It is generally believed that the memory core frequency of 200MHz is the limit of current technology (except for overclocking). DDR technology doubles the data transmission speed. As shown in Figure 2, DDR samples data on both the upper and lower edges of the clock signal. In this way, if the same clock is 200MHz, DDR can reach a data transmission speed of 400Mb/s.
The pre-access technology effectively improves the data transmission speed inside the chip. Prefetch increases the bit width of the DDR memory array. As shown in Figure 3, it is the pre-access process of DDR2 and DDR3. It can be seen that because the pre-access has increased from 4 bits to 8 bits, the same bus At frequency and data rate, the core frequency of DDR3 is half that of DDR2. Lowering the core frequency can reduce power consumption, reduce heat generation, and improve memory stability. And under the same memory core frequency, the bus frequency and data rate of DDR3 is twice that of DDR2.
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