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Microcontrollers play a vital role in embodying intelligence into battery-powered everyday objects to realize the internet of things (IoT). The desirable attributes of such a microcontroller and the like include high energy and area efficiency, and robust error-free operation under dynamic voltage scaling (DVS), workload, process, voltage, and temperature (PVT) variation effects. In this work, a synchronous-logic (S 8051) and a quasi-delay-insensitive asynchronous-logic (A 8051) 8051 microcontroller core are designed and fabricated for full-range DVS from nominal VDD to deep sub-threshold. The performance of the S 8051 and A 8051 are largely comparable at nominal conditions and the entire DVS range, but differs when PVT and workload are varied. At nominal VDD, both the microcontroller cores feature comparable energy and speed, with the electromagnetic interference of the A 8051 ~ 12 dB lower and the area ~ 2 × larger than the S 8051. When DVS is applied, both the microcontroller cores feature comparable energy and speed; the S 8051 requires simultaneous adjustment of clock frequency with VDD. At wide PVT variations, up to ~ 12 × delay margins are required for the S 8051, whereas the A 8051 operates at actual speed. When the workload of both microcontrollers is varied, the A 8051 features lower energy dissipation per workload due to the exploitation of its asynchronous-logic protocols. For IoT applications that incur wide PVT and workload variations, A 8051 is more suitable due to its self-timed nature, whereas when PVT and workload variations are less severe, S 8051 is more suitable due to a smaller IC area.