PVS: passive voltage scaling for wireless sensor networks

Recent wireless sensor nodes, equipped with ultra-low-power (ULP) RISC microcontrollers, do not generally support DVS (dynamic voltage scaling), though the ULP microcontrollers have ideal energy-voltage-frequency characteristics for DVS. In general, an output-adjustable DC-DC converter is hardly all affordable in such sensor nodes, and surprisingly light current consumption makes the DC-DC converter operate in a very inefficient region. This paper introduces a new supply voltage scaling namely PVS (passive voltage scaling) that eliminates the use of a DC-DC converter. While a battery is directly connected to a ULP microcontroller, PVS monitors the battery voltage drop (50% in alkaline batteries) and scales down the clock frequency accordingly for a reliable operation. Opposite to DVS, PVS does not achieve the optimal energy consumption of the microcontroller, but it minimizes the loss of the power delivery system, peripherals and battery, and achieves ultimate longer operational lifetime. Along with such energy gain, the throughput of a PVS-enabled sensor node continuously decreases by the battery state of charge loss, which makes a traditional performance-driven routing also energy-aware. We applied PVS to a wireless sensor network, and demonstrated 34% more network lifetime, and 26% shorter average latency over a modern sensor network such as Telos, 64% more network lifetime and 3% shorter average latency over a traditional sensor network with a DC-DC converter.