Phase enable and clock generation circuit

A phase-enable circuit clocks a first functional unit at a first frequency and a second functional unit at a second frequency. Each of the first and second functional units is provided with a first clock signal of the first frequency. A phase-enable generator then uses the first clock signal and a second clock signal of a second frequency lower than the first frequency to develop a phase-enable signal that periodically disables a clock input terminal of the second functional unit so that the second functional unit is clocked at the second frequency. Changing the frequency of the second clock to zero switches the phase-enable circuit into another mode of operation. In that mode, the clock input terminal of the second functional unit is constantly enabled and the first and second functional units are each clocked at the first frequency.