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Strategies to the Prediction, Mitigation and Management of Product Obsolescence by Michael G. Pecht, Peter Sandborn, Ulrich Ermel, Bjoern Bartels

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5.2 STATIC RANDOM ACCESS MEMORIES (SRAMS)

Static random access memory (SRAM) retains data bits in its memory as long as power is supplied. Once the supply voltage is switched off, all data will be lost, which means that SRAM, like DRAM, belongs to the volatile memories family (non-volatile memories include flash and PROMs, which will be discussed in Section 5.3.1). Unlike DRAMs, which store bits in cells consisting of a capacitor and a transistor, SRAMs do not have to be periodically refreshed. SRAMs store data in a flip-flop consisting of logic transistors and are ready for another read cycle as soon as the first is complete. Unlike SRAMs, DRAMs have to write data back into the cell because data is lost after a read cycle. Hence, SRAMs provide faster access to data than DRAMs. Although SRAMs are faster than DRAMs, the memory capacity of SRAMs is approximately 25 percent of DRAMs for the same chip size and process technology. This is because SRAM transistor structures require more space than capacitors. The cost of memory is largely determined by chip size, so SRAMs cost almost four times more per bit than DRAMs (Garrett, 2000). Figure 5-8 shows that DRAM memory cells only require one transistor and one capacitor, whereas a standard SRAM cell requires six transistors. As for process technology and area requirements on wafers, the DRAM cell is predominant.

FIGURE 5-8 Comparison of DRAM (left) and SRAM (right) structures for a single memory cell (Qin, 2010) (Usenix, 2010).

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