MGV Product Information - DRAM Overview

TYPES OF DRAM

FPM (FAST PAGE MODE) is an operational mode that allows faster memory access by keeping the same row address while selecting random column addresses. This kind of memory is an improvement over older forms of DRAM, making it faster to access data in the same row, or "page." If the data needed is in the same row as the previous data, the memory controller does not have to repeat the row location; it only needs to indicate the next column location. This makes the memory process a little faster. Using FPM memory is like reading a dictionary. As long as the word you want is on the same "page," it will be easy to scroll down the list and find the definition; but when you have to flip pages, it takes a little longer to find what you want.

Example: FPM = 60ns Read Cycle, 60ns Write Cycle, 50MHz operation

EDO (EXTENDED DATA OUT) is almost the same as FPM, with a slight modification that allows back-to- back memory access to occur much faster. Because EDO is easy to implement, it has gained wide acceptance in a very short span of time. A computer system must be designed to support EDO in order to make use of the extra efficiency (10 to 15%) it offers. EDO memory will work in a system that does not support it, but there will be no performance increase. At present, FPM and EDO DRAM make up the majority of main memory for computers. EDO is also called Hyper Page Mode DRAM.

Example: EDO = 20ns Read Cycle, 60ns Write Cycle, 50MHZ operation

SDRAM (SYNCHRONOUS DYNAMIC RANDOM MEMORY) synchronizes all address, data and control signals with a system clock. This technology is a more radical innovation that can synchronize itself with the system clock that controls the CPU. Being "in sync" with the processor eliminates timing delays and makes the memory retrieval process much more efficient.

Example: SDRAM = 10ns Read Cycle, 10ns Write Cycle, 100MHz operation
Note.- The above examples are using 60ns FPM, 60ns EDO, and the SDRAM example is using 10ns, 100MHz that can be faster or slower.

RDRAM (Rambus DRAM) is a completely unique design developed by Rambus, Inc. RDRAM is extremely fast, but requires significant changes in the memory controller and memory/system interface to use. RDRAM uses a narrow, high-bandwidth "channel" to transmit data at speeds about ten times faster than standard DRAM. At present, RDRAM is being used in some game machines and PC graphics applications.

DRAM SPEED

All DRAM manufactured comes off of the lines at different speeds. The DRAMs are tested and stamped accordingly. The average speed of DRAM is 55ns (NanoSeconds). DRAMs that test higher are stamped 70ns and the ones that test lower are stamped 60ns. DRAM can pass at 45ns or 63ns and they are stamped with a 60ns part number.

Although today most systems will work with any speed modules, there are still some systems that are sensitive to speed changes. Older systems with older chip-sets required parity modules. These systems were more sensitive to speed and required complete compliance with the JEDEC specifications. PD (Presence Detect) setting was established to either slow the module down or speed it up. In these systems you cannot miss-match PD sets but can miss-match DRAM speeds used.

Example: 60ns jumpered to 70ns will work fine. 70ns jumpered to 60ns operation will have problems depending on the actual speed of the DRAM as explained above.

Mixing DRAM speed on modules is not good for functionality, but DRAM can be missmatched when generating parity. Parity is a function of your system chip-set and requires a check of DRAM chips that were used. This checking process is not speed sensitive and can be substituted as explained in the above example.

REFRESH RATES

DRAM is made up of electrical cells. These cells must be recharged in rows and thus the refresh rate is 1Krf 2Krf, 4Krf, 8Krf, or Srf detailing how many rows must be refreshed. The refresh rates also establish the speed and power consumption. 2Krf parts can refresh more cells at a time completing the process faster than 4Krf, however, 2Krf uses more power. Srf, Self-Refresh technology enables the components to refresh on their own. Self-Refresh reduces power consumption dramatically and is commonly used in notebook computers.

3.3VOLT VS. 5VOLT DRAM

Making integrated circuits faster and denser requires reduced cell geometry. As components become smaller and smaller, the cell size becomes more compact and more sensitive. As a result, these components cannot withstand the stress of operation at 5-volts. 3.3-volts can operate faster and it also uses less power.

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