The difference between ddr and sdr

At present, the more popular memory modules are mainly these three types: SDR, DDR, RAMBUS. Among them, RAMBUS memory uses a serial connection technology with controlled impedance. We will not discuss it further here. The memory design technology summarized in this article is for SDRAM (including SDR and DDR). Now let's briefly compare SDR and DDR, they are both called synchronous dynamic memory, and their core technology is the same. It's just that DDR has been improved in some functions, so DDR is sometimes called SDRAM II. The full name of DDR is Double Data Rate, which is double the data transfer rate, but its clock frequency has not increased, but the rising and falling edges of the clock can be used to read and write data. For SDR, the common modules on the market are mainly PC100/PC133/PC166, and the corresponding DDR memory is DDR200(PC1600)/DDR266(PC2100)/DDR333(PC2700).

Some specific differences between DDR and SDR:
The voltage and interface circuit are different. SDR generally uses 3.3V LVTTL or SSTL_3 interface, while DDR uses 2.5V SSTL_2 low-voltage digital interface, and DDRII uses 1.8V SSTL_18 interface.
Z A DLL (Delay Locked Loops) is added inside the DDR chip, and the data is accurately located through the data gate DQS (data strobe).
Z DDR can use the rising and falling edges of the clock to read and write data, and its essence is to use the rising and falling edges of the DQS signal to send and receive data.
Z DDR has lower input impedance than SDR.
Z DDR has a reference voltage VREF, the state of the digital signal is determined by the difference between the signal voltage and the reference voltage.
The DM of DDR is only valid during write operations, while the DQM signal in SDR is valid during read operations and write operations.
Z DDR uses differential clock signal transmission, which can better guarantee signal quality and timing.
The interface is different from the motherboard. For example, for DIMM, SDR has 168 pins, while DDR has 184 pins. SODIMM and UDIMM are also different.

Because of the differences in certain characteristics between SDR and DDR, we consider different factors when designing these two types of memory modules. In the following sections, we will elaborate on the design details of these two types of memory modules. Although the current market is still dominated by SDRAM and DDR, major memory module companies have begun to conduct research on DDRII, and JEDEC has also established a special Task Group to start the formulation of new rules. It can be boldly predicted that DDRII will appear on the market next year (2003), and by 2004, DDRII will gradually begin to become the dominant memory market. For the basic knowledge and design of DDRII
The calculation theory can be found in related documents.

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