We present the design of an asynchronous microcontroller core featuring binary compatibility with the MCS 251 family. The work is involved in a specific application project targeting portable electromagnetic measurement systems with very low power consumption. Delay insensitivity was chosen to allow a fast design of complex but safe asynchronous circuits, by automatic synthesis and independency of layout topology. On the other hand, unconventional logic cells must be employed, and special features have been introduced in the synthesis tool to dominate the circuit area, estimating the power consumption and interfacing with more conventional CAD flows. A preliminary set of results is illustrated in detail.