This paper reports the results of heavy ion irradiation of the instruction memory of the VA41630 microcontroller fabricated with the radiation hardened 130nm HARDSIL technology node. The objective of the experiment was to capture single event upsets (SEUs) with precision to accurately estimate the error correcting code (ECC) failure rates in space. The memory exhibited sensitivity to multiple-cell upsets (MCUs), but no single-word multiple-bit upsets (MBUs) during the experiment. Monte Carlo simulation demonstrates that the SEU cross-section can be used to bound the probability of independent upsets leading to ECC failure without consideration of MCUs. However, a geometric analysis concludes that single-word MBUs unobserved in ground testing, may still dominate the ECC failure rates.