Synchronous Dynamic Random Access Memory

Synchronous Dynamic Random Access Memory (engl, short SDRAM, dt " synchronous DRAM ") is a semiconductor memory variant, which is used for example as a memory in your computer.

The short form of SDRAM can also refer to a populated with SDRAM chips DIMM or SO- DIMM circuit board. SDRAM is clocked DRAM technology. The timing is determined by the system, and possibly also by a separate, connected to the system memory bus. The timing is done via the use of registers for address inputs, control information, and the input-output data by value changes in the registers shall be carried out only with the clock edges. By the use of a clock for synchronization eliminates the need for asynchronous communication method (for example, via handshake procedure ). In addition, buffering and pipelining techniques can be used, thus resulting in an overall significant time savings through the use of the registers. SDRAM is about twice as fast as the previous technology EDO DRAM.

Various types of

The widespread as memory types are:

• PC -66 SDRAM: From Intel defined standard, in which the SDRAM will ideally be operated at a speed of 66 MHz. The memory chips soldered on an access time of 12 ns ( rare) or 10 ns.
• PC-100 SDRAM: From Intel defined standard, in which the SDRAM will ideally be operated at a speed of 100 MHz, backward compatible to PC -66 motherboards (operating at 66 MHz is possible). The soldered memory chips have an access time of 8 ns.
• PC -133 SDRAM: With the clock increase the front side bus to 133 MHz VIA forced the operating at the same speed PC -133 SDRAM in order to fully exploit the speed advantage of the clock increase. Backward compatible with PC-66/-100-Hauptplatinen (operating at 66 MHz or 100 MHz possible). The soldered memory chips have an access time of 7.5 ns ( marking on the chips -7.5 rare, but usually a bit irritating -75 ) or 7.0 ns.
• PC-150/166 SDRAM: Particularly powerful SDRAM, which can be operated according to the manufacturer specifications in the eponymous MHz ranges.

PC -66, PC -100 and PC -133 have been specified by the relevant JEDEC committee as standards. In contrast, the PC -150 and PC -166 modules are only overclocked PC -133 modules that have been released by the manufacturer for operation at 150 or 166 MHz.

SDRAM modules were produced in the storage 16 MB, 32 MB, 64 MB, 128 MB, 256 MB, 512 MB ​​and 1024 MB ( rare); usually four, eight or sixteen chips per DIMM have been used. 16 MB modules virtually restricted as a single- sided modules, 32 -MB and 1024 MB modules practically only as a double -sided modules. All other sizes are available both as single- sided as well as double -sided modules.

There are also modules with a CAS latency of two ( CL2) and one of three ( CL3 ), the latter work slightly slower. CL3 modules also often allow operation with CL2 at a lower clock frequency. Thus, for suitable PC -100 CL3 modules can be operated up to 66 MHz clock frequency with CL2, according to PC -133 CL3 modules up to 100 MHz clock frequency with CL2. PC -133 CL2 modules are usually equipped with a memory chip with an access time of 7.0 ns.

Registered SDRAM

Registered SDRAM has nothing to do with the registration of the SDRAM DIMM specification data in the SPD EEPROM ( Serial Presence Detect) to do on the memory module is often confused with the function of " registered". Learn more about this see under Registered module.

As a Registered SDRAM SDRAM modules are referred to, which are equipped with a register for the address and control lines. Registered SDRAM DIMMs thus reduce the load ( fan-out ), they cause for the motherboard so that larger and more DIMMs can be used. This is a widely used technique for servers to increase the maximum memory size. On a Registered SDRAM DIMM can be accessed more slowly than corresponding unbuffered modules ( unbuffered ).

Buffered / unbuffered SDRAM

SDRAM DIMMs maximum power comprise a plurality of modules ( chips), and have for this reason, in the current normal high clock rates, higher capacitive and inductive loads to the address and control lines as compared to the SDRAM DIMMs with smaller memory size. Therefore, some board designers set double driver buffer to the SDRAM DIMM, so as to amplify the signals on the lines and to reduce the system load compared to otherwise identical memory modules with these additional output buffers. However, these buffers cause a small time delay of electrical impulses, so adding such a buffer to a normally crowded module without buffers leads to a slowing of the signals compared to the same module with output buffers. This is a also mainly used in server technology, to increase the maximum possible amount of memory on a system board ( motherboard ).

Operating voltage

SDRAM memory chips require a supply voltage of 3.3V.

Compatibility issues

PC133 modules from more recent production, may be incompatible with the early memory controllers with SDRAM support. So it happens that newer PC133 modules do not work properly on older motherboards, although the DIMMs are still with respect to their total storage capacity within the chipset or motherboard specifications. A typical example is 256MB PC133 modules on Super Socket 7 motherboards with the chipset VIA Apollo MVP3. While older DIMMs, double-sided equipped with eight 128Mbit chips on such motherboards are working properly, newer, one-sided with eight 256Mbit chips stocked 256MB memory modules do not work or are only recognized as 128MB DIMM. In addition to the storage density can affect the compatibility to the memory controller and an unfavorable internal organization of the SDRAM chips used. 512MB and 1024MB modules do not work on motherboards with VIA's Apollo MVP3 chipset. One reason may be too high capacitive load caused by too many parallel memory cells that overwhelm the drivers and leads to soft clock edges.

Technical development

If value changes in both positive and at negative clock edges are possible, then one speaks of DDR SDRAM ( Double Data Rate SDRAM). DDR SDRAM is the advancement of SDRAM technology dar. to linguistic demarcation the first generation of SDRAM technology today SDR- SDRAM ( Single Data Rate SDRAM) is called. While SDR- SDRAM DIMMs have only 168 pins DDR SDRAM modules have 184 pins.