We present a novel low cost scheme for the on-die measurement of either clock jitter, or process parameter variations. By re-using and properly modifying the Ring Oscillators (ROs) that are currently widely employed for process parameter variation measurement in high performance microprocessors, our proposed scheme can be easily set in either the process parameter variation measurement mode, or the clock jitter measurement mode, by acting on an external control signal. This way, during the test or debug phase, clock jitter can also be measured at negligible area and power costs with respect to process parameter variation measurement only. Our scheme is scalable in the provided clock jitter measurement resolution, while allowing the same process parameter variation measurement resolution as the currently employed RO based schemes. Moreover, due to its allowing both process parameter variation and clock jitter measurements, our scheme features accurate clock jitter measurement despite the possible presence of significant process parameter variations.