Can this device work from 0.9 - 3.6 V? Why do you mention two specific operating ranges: 0.9-1.8 V and 1.8 - 3.6 V?
The C8051F9xx devices were designed to be powered from unregulated batteries, and no commonly-used battery cells or combinations of cells provide voltages between 1.6 V and 1.8 V at any point during their useful operating life. The C8051F9xx devices are designed to provide full analog and digital performance and minimum power consumption at an internal supply voltage of 1.8 V, so we provide two distinct operating modes with different pin connections and external components, based on whether the battery voltage is above or below 1.8 V.
Most of today’s low-power products use alkaline, carbon-zinc, silver oxide, NiCd, or NiMH battery chemistries, all of which have cell voltages that range from 1.2–1.6 V when fresh, and from 0.9–1.0 V at the end of their useful life. Two of these cells in series will provide a supply voltage between 1.8 V and 3.2 V; therefore, the C8051F9xx “Two-Cell Mode” supply voltage range of 1.8 V to 3.6 V can accommodate two standard battery cells, a single lithium coin cell (having an operating voltage range of 3.0–2.0 V), and regulated supplies ranging from 2–3.3 V, ±10 %.
One feature that separates the C8051F9xx family from all other general-purpose, 8-bit microcontrollers is the ability to operate from a single-cell battery at voltages down to 0.9 V. In one-cell mode, some circuit blocks operate directly from the battery; however, the digital core and most analog peripherals are powered from an inductor-based dc-dc converter that generates a supply voltage above 1.8 V. Since the one-cell and two-cell modes require different pin connections and different external components, we specify separate supply voltage ranges for each.