C H A P T E R 1 3 • Memory Devices and Systems
In recent years, memory has become one of the most important topics in digital electronics. This is tied closely to the increasing prominence of cheap and readily available microprocessor chips. The simplest memory is a device we are already familiar with: the D flip-flop. This device stores a single bit of information as long as necessary. This simple
concept is at the heart of all memory devices.
The other basic concept of memory is the organization of stored data. Bits are stored in locations specified by an “address,” a unique number which tells a digital system how to find data that have been previously stored. (By analogy, think of your street address: a unique way to find you and anyone you live with.)
Some memory can be written to and read from in random order; this is called random access read/write memory (RAM). Other memory can be read only: read only memory (ROM). Yet another type of memory, sequential memory, can be read or written only in a specific sequence. There are several variations on all these basic classes.
Memory devices are usually part of a larger system, including a microprocessor, peripheral devices, and a system of tristate busses. If dynamic RAM is used in such a system, it is often in a memory module of some type. The capacity of a single memory chip is usually less than the memory capacity of the microprocessor system in which it is used. In order to use the full system capacity, it is necessary to use a method of memory address decoding to select a particular RAM device for a specified portion of system memory.
13.1 Basic Memory Concepts
K E Y T E R M S
Memory A device for storing digital data in such a way that they can be recalled for later use in a digital system.
Data Binary digits (0s and 1s) that contain some kind of information. The digital contents of a memory device.
Address A number, represented by the binary states of a group of inputs or out-
puts, uniquely defining the location of data stored in a memory device.
Write Store data in a memory device.
Read Retrieve data from a memory device.
Byte A group of 8 bits.
Nibble Half a byte; 4 bits.
octal_latch.gdf 1x8mem.gdf
1x8mem.scf
Address and Data
A memory is a digital device or circuit that can store one or more bits of data. The sim-
component in a MAX PLUS II graphic file and configured as an 8-bit memory.
When the WRITEn line goes LOW, then HIGH, data at the DATA_IN are stored in the eight latches. Data are available at the DATA_OUT pins when READ is HIGH. Note that although the READ and WRITEn inputs are separate in this design, their functions would often be implemented as opposite logic levels of the same pin.
Figure 13.4 shows a simulation of the 8-bit memory. The LOW pulses on WRITEn write the data, shown as two hexadecimal digits on the DATA_IN line, into the latches. To read the values stored in the eight latches, we set READ HIGH. In between read states, all DATA_OUT lines are in the high-impedance state, indicated by the notation ZZ.