Random Access Memories


Posted on Feb 4, 2014

The memory system design: the detailed timing waveforms for a static RAM component, and the design of the register and control logic that surrounds the memory subsystem, making it possible to interface the memory to the rest of the digital system. But first, we must begin with the basics. We begin with a relatively simple memory component, a 1024 by 4-bit static RAM. The basic storage element


Random Access Memories
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of the static RAM is a six-transistor circuit, shown in Figure 7. 46. The "static" storage element is provided by the cross-coupled inverters. This circuit configuration will hold a 1 or 0 as long as the system continues to receive power. There is no need for a periodic refreshing signal or a clock. . To write the memory element, special circuitry in the RAM drives the data bit and its complement onto these lines while the word enable line is asserted. When driven in this fashion, the data bit can overwrite the previous state of the element. To read the contents of the storage element, the word enable line is once again enabled. Instead of being driven onto the data lines (also called bit lines), the data are "sensed" by a different collection of special circuits. These circuits, called sense amplifiers, can detect small voltage differences between the data line and its complement. If Dataj is at higher voltage than RAMs are efficient in packing many bits into a circuit package for two reasons. First, only a small number of transistors are needed to implement the storage elements. And second, it is easy to arrange these elements into rows and columns. Each row of memory cells shares a common word enable line. Each column shares common bit lines. The number of columns determines the bit width of each word. Thus, you can find memory components that are 1, 4, or 8 bits wide and that read or write the bits of a single word in...




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