Programmable logic devices (PLDs) come in a variety of forms including programmable array logic (PAL), programmable read-only memory (PROM), and programmable logic array (PLA). Each of these devices allows the particular user a variety of input and output features, the choice of any particular type of device is dependent upon the particular application.
In all programmable logic devices, common design objectives exist. Specifically, it is desirable for PLD manufacturers to provide the maximum of programming features in a single device utilizing a minimum amount of power to provide those features.
One class of programmable logic devices are comprised of a plurality of input lines, an array of AND gates, and an array of OR gates, with either the AND array, the OR array, or both being programmable. In a PAL, for example, the AND array is programmable and the OR array is fixed. Each array generates logic signal outputs; typically, the outputs of the AND array (referred to as product terms) comprise inputs to the OR plane. Thus, in a PAL, the output of each programmable AND gate in the array of a
comprises a product term input to the OR plane.
In conventional PALs, each product term drives one input of an OR or NOR gate. In large programmable arrays, it is desirable to program the input term/product term junctures with a minimum amount of delay. In order to accomplish this, a sense amplifier is coupled at the output of the product term lines to act as a threshold detector. The sense amplifier detects minute changes in voltage on the product term line and outputs a magnified reflection of the threshold change signifying a programmed product term. The output of the sense amplifier thus comprises a high/low input to the OR plane and allows programming of large AND arrays with a minimum amount of delay.
Typically, a programmer will automatically determine which product terms in the array are not required for the particular device application. Such unused product terms must nonetheless be programmed to a particular state to ensure that the unused term does not have an adverse effect as an input to the OR or NOR arrays. Thus, each of the sense amplifiers coupled to a particular product term output constantly draws power in the programmable logic device, whether or not the product term to which it is coupled is used in the application. The power consumed by the sense amplifiers of unused product terms comprises a significant portion of the total power requirement for a particular programmable logic device. The power drawn by the sense amplifier of an unused product term is essentially wasted.