A free running oscillator (FRO) includes a reference current generator, a current converter, and first and second oscillator cores. The reference current generator generates a first current. The current converter generates a second current based on the first current. The first oscillator core generates a clock signal at a first frequency based on a first value of the second current. The second oscillator core generates a clock signal at a second frequency based on a second value of the second current. The second frequency may be lower than the first frequency, and the second value of the second current lower than the first value of the second current.