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Programmable Devices
A programmable logic device (PLD) is an electronic component used to build reconfigurable digital circuits. Unlike a logic gate, which has a fixed function, a PLD has an undefined function at the time of manufacture. Before the PLD can be used in a circuit it must be programmed, that is, reconfigured. Before PLDs were invented, read-only memory (ROM) chips were used to create arbitrary combinational logic functions of a number of inputs. Consider a ROM with m inputs (the address lines) and n outputs (the data lines). When used as a memory, the ROM contains 2m words of n bits each. Now imagine that the inputs are driven not by an m-bit address, but by m independent logic signals. Theoretically, there are 22m possible Boolean functions of these m input signals. By Boolean function in this context is meant a single function that maps each of the 2m possible combinations of the m Boolean inputs to a single Boolean output. There are 22m possible distinct ways to map each of 2m inputs to a Boolean value, which explains why there are 22m such Boolean functions of m inputs. Now, consider that each of the n output pins acts, independently, as a logic device that is specially selected to sample just one of the possible 22m such functions. At any given time, only one of the 2m possible input values can be present on the ROM, but over time, as the input values span their full possible domain, each output pin will map out its particular function of the 2m possible input values, from among the 22m possible such functions. Note that the structure of the ROM allows just n of the 22m possible such Boolean functions to be produced at the output pins. The ROM therefore becomes equivalent to n separate logic circuits, each of which generates a chosen function of the m inputs.
