Avionic systems based on next generation microprocessors will be mixed voltage designs. Next generation microprocessors have a low voltage power supply and interface, but most of the avionics-grade support logic will remain standard voltage devices in the near term. The mixture of integrated circuit voltage technologies poses some unique problems to the avionics designer. The paper will present some fundamental techniques for designing mixed voltage, microprocessor-based avionics systems. Included will be techniques for interfacing devices of different voltage technologies and the creation of an accurate low voltage power supply.