in a specialized functional region of a programmable logic device , in which certain components may not be used , those components can be placed in a low - power mode so that they do not switch . for example , in an adder which is not being used but is receiving inputs , the current path for the adding circuitry is interrupted , while the output is forced low . if the adder is a carry / look - ahead adder , the generate and propagate signals normally used in subsequent stages to predict the value of the carry signal are forced to constant values even if the inputs to the adder are changing .