Simple design for UP/DOWN Gray-code counters

A simple design for a presettable UP/DOWN Gray-code counter is described. The design is such that the logic requirements for the UP mode are identical to those for the DOWN mode, a single AND/OR gate being the only addition to the entire counter. Furthermore, since the logic between any two consecutive stages remains unaltered, the counter can be readily extended for larger numbers of bits. A typical application of the UP/DOWN feature is in Gray-code addition/subtraction.