The increased performance from technology scaling makes it feasible to operate digital circuits at ultra-low voltages without the significant performance limitation of earlier process generations. The theoretical minimum energy point resides in near-threshold voltages in current processes, but device and environment variations make it a challenge to operate the circuits reliably. This paper presents an ASIC implementation of a 32-bit RISC CPU in 28nm CMOS employing timing-error prevention with clock stretching to enable it to operate with minimal safety margins while maximizing energy efficiency. Measurements show 3.15pJ/cyc energy consumption at 400mV/2.4MHz, which corresponds to 39% energy savings and 83% EDP reduction compared to operation based on static signoff timing.