Programmable Logic Devices:Programmable Array Logic

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Programmable Array Logic

A programmable array logic (PAL) is a special type of a PLA where the OR array is not programmable. In other words, in a PAL, the AND array is programmable but the OR array is fixed; whereas in a PLA, both arrays are programmable. The advantage of PALs is the elimination of fuses in the OR array in Figure 45.1(a) and special electronic circuits to blow these fuses. Since these special electronic circuits and programmable OR array occupy a very large area, the area is significantly reduced in PAL. Since single-output, two-level networks (i.e., many AND gates in the first level and one OR gate as the network output) are needed most often in desing practice, many single-output two-level networks which are mutually unconnected are placed in some PAL packages.

In digital systems, many non-standard networks are still used because designers want to differentiate their computers from competitors’. But logic functions that designers want to have are too diverse to be standardized by semiconductor manufacturers. When off-the-shelf IC packages for standard networks, including microprocessors and their peripheral networks, are assembled on pc boards, many non- standard networks are usually required for interfacing them to other key networks or for minor modifications. So, they require many discrete components and IC packages, each of which has a smaller number of transistors, in addition to a microprocessor package with millions of gates, occupying a significant share of the areas on pc boards. Now, we can make connections inside PALs, instead of custom-making pc boards. Custom-made pc boards are expensive and time-consuming because connection patterns on pc boards need to be designed, these pc boards need to be manufactured and then the holes of pc boards have to be soldered to the pins of IC packages. The replacement by PAL packages can substantially reduce the area, time, and cost. If we consider related factors such as reductions of cabinet size, power consumption, and fans, the significance of this reduction is further appreciated.

There are mask-programmable PALs and field-programmable PALs (i.e., FPALs). When logic design is not finalized and needs to be changed often, FPAL packages can reduce expense and time for repeatedly redesigning and remaking pc boards.

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