A device and a method for generating a output clock signal having a output cycle, the method includes: (i) adjusting a delay of an adjustable ring oscillator to provide a high frequency clock signal having a short cycle so that the output cycle substantially equals a sum of integer multiples of a sleep cycle and integer multiplies of the short cycle; wherein the output cycle differs from any integer multiples of the sleep cycle; wherein the sleep cycle characterizes a sleep clock signal that is generated by a low frequency sleep clock; wherein the short cycle is shorter than the sleep cycle; (ii) counting short cycles and sleep cycles; and (iii) generating, during a sleep mode, in response to the counting and to a predefined counting pattern, the first clock signal; wherein the generating includes activating the adjustable ring oscillator only during a portion of a single sleep cycle per each output cycle.