  This paper presents a novel software driven voltage tuning method that
utilises multi-purpose Ring Oscillators (ROs) to provide process variation and
environment sensitive energy reductions. The proposed technique enables voltage
tuning based on the observed frequency of the ROs, taken as a representation of
the device speed and used to estimate a safe minimum operating voltage at a
given core frequency. A conservative linear relationship between RO frequency
and silicon speed is used to approximate the critical path of the processor.
  Using a multi-purpose RO not specifically implemented for critical path
characterisation is a unique approach to voltage tuning. The parameters
governing the relationship between RO and silicon speed are obtained through
the testing of a sample of processors from different wafer regions. These
parameters can then be used on all devices of that model. The tuning method and
software control framework is demonstrated on a sample of XMOS XS1-U8A-64
embedded microprocessors, yielding a dynamic power saving of up to 25% with no
performance reduction and no negative impact on the real-time constraints of
the embedded software running on the processor.
