A control circuit comprising a read-only memory (3) has its storage area divided into two areas each containing the same number of addressable words. By arranging that input of a single address signal (An+1) causes a change from addressing a particular word in one area to addressing a corresponding word in the other area, and by programming a fixed difference in value between corresponding words in the two areas causes a step function in the output. Using a digital to analogue converter (1) on the output of the circuit permits a step change in a voltage output (1) to be provided to (eg) a voltage controlled oscillator (4). A control circuit using a read-only memory as a voltage comparator is also disclosed.