The SEU vulnerability of the SA3300 16-bit microprocessor has been characterized, and the effects of two different design revisions on error rate have been explored. We found that the threshold for upset depends on the data pattern written into the general purpose registers. With all bits in the general purpose registers set to logic one, a design with 2-pm n-. and p-channel transistor lengths had a threshold LET of 35 MeVcm2/mg at 25°C and 4.5 volt operation. With all zero's stored in the registers the upset threshold increased by more than a factor of two to 83 MeVcm2/mg. A second design revision, with 1.25-pm and 1.75-pm n-and p-channel transistor lengths, respectively, was more vulnerable to upset, but exhibited a smaller dependence on logic state. Measured threshold LET was 23 and 35 MeVcm2/mg with all one's and all zero's, respectively. Microprobe measurements using a pulsed Nd:YAG laser suggest that the observed pattern dependence for both design revisions is due to bipolar photocurrent in a vertical n+pn transistor. A slight temperature dependence was observed in both design revisions. This is consistent with the use of oversized restoring transistors to minimize SEU vulnerability rather than polysilicon feedback resistors. More recent data show thresholds above 120 MeV-cmz/mg with 80 kn feedback resistors.