Power supply variations and jitter are measured by monitoring the performance of a ring oscillator on a cycle-by-cycle basis. Performance is measured by counting the number of stages of the ring oscillator that are traversed during the clock cycle and mapping the number of stages traversed to a particular voltage level. Counters are used to count the number of ring oscillator revolutions and latches are used to latch the state of the ring oscillator at the end of the cycle. Based on the counters and latches, a monitor output is generated that may also incorporate an adjustment for a reset delay associated with initializing the ring oscillator and counters to a known state.