SDRAM (Synchronous Dynamic Random Access Memory) is a name for various types of memory in which input and output signals are synchronised with the incoming memory clock. The speed of SDRAM is rated in MHz rather than in nanoseconds so it can be easily compared to the system bus speed. There are various types/standards of SDRAM available today and inside every type there are different sub-types where the main differences are power consumption, speed and data bus size. Use of any type of SDRAM requires a complex DRAM controller, which is normally part of the microcontroller.
SDRAM is very often used as a synonym for Single Data Rate SDRAM or SDR SDRAM.
Single Data Rate SDRAM accepts only one command and transfers one word of data per one clock cycle, with typical clock frequencies of 100 and 133 MHz. All control signals are sampled on the raising clock edge.
The most common data bus sizes are 4, 8, 16 or 32 bits and multiple chips can be combined together to provide larger data bus sizes. SDRAM devices are divided into 2 or 4 internal memory banks, which can be accessed independently.
SDRAM operating voltage is generally 3.3V. However, there are so called Mobile SDRAM devices available with operating voltage of 1.8V, 2.5V and 3.3V.
Although the SDR SDRAM components are fast when compared with past technologies they are nowhere near the speeds required by today's very fast computer CPUs. However, SDR SDRAM devices still have their place in embedded system designs where bus speeds of 100 MHz or 133 MHz are required.
The major manufacturers of SDRAM devices are Samsung Electronics, Micron Technologies, Qimonda and Hynix.
Double Data Rate SDRAM or DDR is similar to SDR SDRAM. The major difference and significant improvement that DDR provides is the ability of DDR to transfer the data at both edges of the clock signal, which effectively doubles the transfer rate (bandwidth) with the same bus speed as with typical SDR SDRAM.
The standard DDR speeds are DDR-200 (100 MHz memory clock), DDR-266 (133 MHz), DDR-333 (166 MHz) and DDR-400 (200 MHz).
The data bus sizes are similar to SDR SDRAM (4, 8, 16 or 32 bits) and multiple chips can be combined together to provide larger data bus sizes.
DDR operates at 2.5V compared to 3.3V for SDR SDRAM. This is a significant power reduction which can be further decreased by using Mobile DDR devices running at 1.8V.
For similar reasons to those above for SDR devices, DDR memory is being superseded by DDR2 SDRAM which operates on the same principle as DDR (although, there are some modifications required in order to support higher speeds/clock frequency). Today, DDR SDRAM devices are used in systems (embedded systems, mobile devices, PDAs, etc.) where higher bandwidth is required, but where the front-size bus speed limitations, higher latency and/or the cost prohibit the use of DDR2 devices.
DDR2 and DDR3 SDRAM
Double Data Rate 2 SDRAM or DDR2 is the current standard in the memory industry and it is used mostly in the computer industry. The main advantage compared to the older DDR technology is the speed - the DDR2 memory devices run their bus at twice the speed of its memory cells. This enables faster front side bus speeds and higher peak throughputs. However, those speed improvements are achieved at the expense of higher latency (the delay incurred when a fast CPU tries to access data in SDRAM, usually measured in memory bus clock cycles). It should be noted that, because of this higher latency, DDR SDRAM is considered superior to DDR2 when both are running at the same bus speed (DDR2 read latency can be anywhere between 3 and 9 cycles, whereas DDR read latency is 2 to 3 cycles).
The standard DDR2 speeds are DDR2-400 (100 MHz memory clock), DDR2-533 (133 MHz), DDR2-667 (166 MHz) and DDR2-800 (200 MHz). DDR2 devices operate at 1.8V.
Double Data Rate 3 SDRAM or DDR3 is the newest member of SDRAM family. It runs its I/O bus at four times the speed of its memory cells and has about 30% reduced power consumption compared to current DDR2 technology thanks to lower operating currents and voltages (1.5V).
The standard DDR3 speeds are DDR3-800 (100 MHz data bus speed), DDR3-1066 (133 MHz), DDR3-1333 (166 MHz) and DDR3-1600 (200 MHz). DDR3 devices operate at 1.5V.
Both DDR2 and DDR3 devices are rarely used in electronic devices other than the computer systems and other devices which require very fast transfer of information (graphic or video intensive applications).
Bluewater Systems Experience
Both SDR and DDR SDRAM technologies are widely used in the products designed by Bluewater Systems, including 16- and 32-bit wide data bus with memory clock of up to 133 MHz (i.e. SDR 133 and DDR-266). 'Mobile' SDRAM is used wherever the microcontroller supports it.