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Phoenix: an Ultra-Low Power Processor for Cubic Millimeter Sensor Systems

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The University of Michigan developed a low power chip for cubic millimeter sensors in 2009. It was built in 180 micron lithography process so the chip could theoretically be reduced in size by about 20 times with the latest lithography process. However, the key is power management.

An integrated platform for sensor applications, called the Phoenix Processor, is implemented in a 0.18μm CMOS process with an area of 915×915 microns, making on-die battery integration feasible. The Phoenix Processor uses a comprehensive standby strategy with a unique power gating approach, custom low leakage memory cells, low voltage ROM, CPU with compact ISA, data memory compression, and adaptive data memory leakage management. Measurements show that the Phoenix Processor consumes on average 29.6pW in standby mode and 2.8pJ/cycle in active mode.

One cubic millimeter systems will deliver enormous value to a wide range of wireless sensor applications. While general environmental sensor systems will benefit from the lower costs associated with small volume, cubic millimeter systems will be particularly beneficial for medical implants such as intra-ocular pressure sensors by making surgery less invasive.

Although circuit component size can be reduced due to advances in semiconductor technology, the required battery size for a target lifetime is the major bottleneck to achieving one cubic millimeter system volume. Given a 1mm2 zinc/silver battery with a capacity of 100uAh/cm2 and output voltage of 1.55V, system power consumption should be limited to 177pW to guarantee one year of battery life.

This power source limitation provides motivation for a system that consumes extremely little power. This paper extends upon the state-of-the-art by presenting an ultra-low power system, called the Phoenix Processor, which consumes only 29.6pW in standby mode. With such low power demands, a 1mm2 battery could supply power for a multi-year lifetime. The system includes data RAM (DMEM), instruction RAM (IMEM), instruction ROM (IROM), CPU, power management unit (PMU), watchdog timer, and temperature sensor.

The core which serves as a parent to the peripherals consists of an 8-bit CPU, a 52×40-bit DMEM, a 64×10-bit IMEM, a 64×10-bit IROM and PMU. The peripherals include a watchdog timer and a temperature sensor,
but can be extended up to 8 devices for sensing system requiring additional functionality. The core and peripheral devices communicate over a system bus using a simple asynchronous protocol

In a typical sensing application that requires 2000 instructions to be run once every 10 minutes, the average power is only 39pW.

In this work, we describe a sensor processor that operates at Vdd=0.5V to minimize active mode energy and uses device-, circuit-, and architecture-level techniques to minimize standby mode energy. Measurements show that Phoenix Processor consumes 297nW in active mode and only 29.6pW in standby
mode. A thin film or micro-sized battery with the same form factor as Phoenix Processor could provide a lifetime on the order of years, making Phoenix Processor an attractive candidate for future sensor type computing systems.

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