One feature pertains to a method that includes implementing a Physical Unclonable Function (PUF) circuit, and obtaining a first set of output bits from the PUF circuit by operating the PUF circuit at a first supply voltage level and/or first frequency. Then, at least one of the first supply voltage level is changed to a second supply voltage level and/or the first frequency is changed to a second frequency, where the second supply voltage level and the second frequency are different than the first supply voltage level and the first frequency, respectively. A second set of output bits is then obtained by operating the PUF circuit at the second supply voltage level and/or the second frequency, where the second set of output bits is in part different than the first set. Secure data is generated using the first set of output bits and the second sets of output bits.