We improve entropy bounds for a self-timed ring based true random number generator, taking the timing of the reference clock signals into account. The models we discuss encompass both perfect and jittered reference clocks. Importantly, our novel analysis of jittered reference clocks can be used to study how robust the improved entropy bounds are. We use parity filters as post-processing blocks and improve results on the required minimal parity filter size to obtain a given target entropy. In addition, we see in numerical experiments that these models are robust in the sense that the minimal required size of a parity filter to exceed a given entropy bound does not change when weakening the assumption on the reference clock; i.e. when considering jittered instead of perfect reference clocks.
