When someone says “SDRAM” they might mean one of two things. It could be be referring to SDR SDRAM (also known as legacy SDRAM) or DDR SDRAM. If this sounds confusing, don’t worry, we’ll clear it up in this article.
SDR SDRAM, like it’s younger cousin DDR SDRAM and even the most modern version DDR5, are a type of computer memory know as DRAM.
The idea of DRAM is to allow random access semiconductor memory to store every bit of data in a memory cell. The research for this design dates back all the way to the 1940s with the invention of The Williams Kilburn Tube.
SDR SDRAM was the standard from the 1970s through the early 1990s. SDR SDRAM integrated circuits were a revolution at the time because they allowed the memory circuit to synchronize with the externally supplied clock signal from the CPU. This means that the CPU and the RAM chip could now work without having to wait for each process to finish. The standard helped create the JEDEC, which is currently responsible for coordinating microchip processing standards.
The first big difference between SDR SDRAM and DDR SDRAM is that the SDR version does a single write per cycle while DDR does two writes. In fact, the DDR stands for Double Data Rate. The reason it can do this is that clock cycles have two parts; the leading edge and the trailing edge. Legacy SDRAM only wrote to the leading edge. Because DDR writes both during the leading and trailing edge, if the frequency, latency, clock rate and cycle time were the same, DDR would be twice as fast. But DDR is more than just twice as fast. This is because DDR SDRAM is generally created at higher frequencies which make the first generation of DDR over 3x faster than the last generation of SDR SDRAM.
Another big part of the development of SDR vs DDR is the voltage.
Legacy SDR generally ran at 3.3 Volts while DDR SDRAM runs at 2.6 V and the latest version of DDR, DDR5 runs at 1.1 Volts. This gives chip manufacturers a huge advantage in heat dissipation. When clocking computer chips, heat is a big limiter of how far you can push the envelope. By having lower voltage chips, chip manufacturers can take advantage of the chips capacity without as much worry about producing heat which can hurt the chip and reduce the efficiency of the flow of electricity.
Legacy SDR was created at a time when chip frequency was in the 66MHz to 133MHz range. When DDR technology was create the first DDR computer memory chips had a frequency of 200MHz with the highest level gen 1 DDR version reaching up to 400MHz. Another interesting change as the number of pins. The old chips had only 168-pin DIMM modules, while the newer ones accommodated more portable devises by integrating not only DIMM buffered modules but also SODIMM and even MicroDIMM. Interesting enough the latest iteration of the DDR ram chips, DDR5, are specifically geared toward portable devises and therefore are SODIMM focused first. This is a giant transition away from Desktop and Server focused chip development that dominated in the past.