*5.1. NTV-CPU Results*

The NTV Processor is fabricated in a 32-nm CMOS process technology with nine layers of copper interconnect. Figure 12a shows the IA-32 die and core micrographs with a core area of 2 mm2. Figure 12b shows the packaged IA processor and the solar cell (1 square inch area) used to power the core. The IA core is operational over a wide voltage range from 280 mV to 1.2 V. Figure 13 shows the measured total core power and maximum operational frequency (Fmax) across the voltage range, measured while running the Pentium Built-In Self-Test (BIST) in a continuous loop mode. Starting at 1.2 V and 915 MHz, core voltage and performance scales down to 280 mV and 3 MHz, reducing

total power consumption from 737 mW to a mere 2 mW. With a dual-VDD design, memories stay at its measured VDD-min of 0.55 V while allowing IA core logic to scale further down till 280 mV.

**Figure 12.** NTV-CPU: (**a**) Full-Chip 32-nm die and core micrographs and characteristics; (**b**) Packaged IA-32 silicon and the small solar cell used to power the core.

**Figure 13.** NTV-CPU measured power, performance and energy characteristics across wide VDD.

Figure 13 also plots the measured total energy per cycle across the wide voltage range along with its dynamic and leakage components. Minimum energy operation is achieved at NTV, with the total energy reaching minima of 170 pJ/cycle at 450 mV (VOPT), demonstrating 4.7× improvement in energy efficiency compared to the VDD-max (1.2V) corner.

The pie-charts in Figure 14 shows a total core power breakup across super-threshold, near-threshold and sub-threshold regions. The contribution of logic dynamic power reduces drastically from 81% at VDD-max to only 4% at VDD-min (280 mV). Chip leakage power contribution as a proportion of total power starts increasing in the near-threshold voltage region and accounts for 42% of the total core power at VDD-opt. At VDD-min point, memories continue to stay at a higher VDD than logic (550 mV), thus contributing 63% of the total core power.

**Figure 14.** Measured NTV-CPU power breakdown across wide voltage range. Note that the memory supply scales down to 550mV, while the core logic operates well into the sub-threshold regime.
