IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 44, NO. 4, APRIL 2009 1145 A Low-Voltage Processor for Sensing Applications With Picowatt Standby Mode Scott Hanson, Student Member, IEEE, Mingoo Seok, Student Member, IEEE, Yu-Shiang Lin, Member, IEEE, ZhiYoong Foo, Daeyeon Kim, Student Member, IEEE, Yoonmyung Lee, Student Member, IEEE, Nurrachman Liu, Student Member, IEEE, Dennis Sylvester, Senior Member, IEEE, and David Blaauw, Senior Member, IEEE Abstract—Recent progress in ultra-low-power circuit design is creating new opportunities for cubic millimeter computing. Robust low-voltage operation has reduced active mode power consump- tion considerably, but standby mode power consumption has re- ceived relatively little attention from low-voltage designers. In this work, we describe a low-voltage processor called the Phoenix Pro- cessor that has been designed at the device, circuit, and architec- ture levels to minimize standby power. A test chip has been im- plemented in a carefully selected 0.18 m process in an area of only 915 915 m . Measurements show that Phoenix consumes 35.4 pW in standby mode and 226 nW in active mode. Index Terms—Low voltage, ultra-low leakage, ultra-low power. I. INTRODUCTION T HE prevalence of mobile computing has helped define a vision of complex computational resources in miniscule volumes [1], [2]. As the volumes of computing resources approach one cubic millimeter, active monitoring and actuation can be used to enrich a wide range of applications. Cubic mil- limeter computing will be particularly important in implantable medical devices, where reducing device volume helps minimize implant damage to the body. The diagnosis and treatment of Glaucoma, for example, requires periodic measurements of pressure in the eye (intra-ocular pressure). Intra-ocular pressure is currently monitored directly by a doctor, requiring frequent trips to the doctor’s office to ensure sufficient temporal resolu- tion [3]. An intra-ocular pressure sensor with a MEMS pressure sensor, microprocessor, memory, radio and power source small enough to be implanted in the eye would reduce both cost and time investment and would increase the temporal resolution of pressure measurements. Although MEMS and circuit components easily meet the volume constraints of intra-ocular pressure sensing and other cubic millimeter computing applications, batteries and energy scavenging power sources cannot be easily miniaturized while Manuscript received August 25, 2008; revised November 28, 2008. Current version published March 25, 2009. S. Hanson, M. Seok, Z. Foo, D. Kim, Y. Lee, N. Liu, D. Sylvester, and D. Blaauw are with the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: han- sons@umich.edu). Y.-S. Lin was with the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109 USA. He is now with the IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSSC.2009.2014205 also serving the power demands of the MEMS and circuit com- ponents. Minimizing the power demands of each component is therefore one of the central challenges in designing a cubic millimeter computing system. Consider a system with a thin film zinc/silver oxide battery with a capacity of 100 Ah/cm and output voltage of 1.55 V [4]. If the battery size is restricted to 1 mm , the average system current must be only 114 pA (for power consumption of 177 pW) to guarantee one year of battery life. For the remainder of this work, we explore power mini- mization in digital components including the microprocessor, memory, watchdog timers and simple sensors. A growing body of work has studied the use of ultra-low-voltage operation to minimize active mode power consumption in digital circuits. Early work proved that operation at extremely low voltage was possible [8], and later work showed that memory can be redesigned to operate robustly at low voltage [13]–[15]. The authors of [5], [7], [21] demonstrated that unprecedented energy efficiency can be achieved in general purpose subthreshold pro- cessors. A number of well-known techniques have also been used widely to reduce standby power including the use of high- devices, clock gating, and power gating. As an alternative to power gating, the ultra-low-power processor presented in [23] operated at a lower voltage during standby. The subthreshold processor in [21] applied a similar technique to SRAM during standby but implemented clock gating and power gating on logic as well to achieve standby mode power under 1 W. We have developed an ultra-low-energy sensor processor called the Phoenix Processor that leverages low-voltage opera- tion and several well-known standby mode techniques. Unlike prior work, we choose standby power as the primary design metric. We have implemented a much more aggressive standby strategy than past low-voltage processors [5], [7], [21]. This is of particular importance in a typical wireless monitoring system that spends the majority of its lifetime in standby mode. For example, a temperature logger might take sensor measure- ments once every 10 minutes. Assuming a 100 ms active period and a low-voltage processor that consumes 5 more power in active mode than standby mode [5], the temperature logger consumes 1200 more energy in standby mode than active mode. To address this discrepancy in Phoenix, we have implemented an aggressive standby mode strategy including the deliberate selection of an older low-leakage technology, an alternative power gating approach, a custom leakage-optimized instruction set, simple data memory compression, and a new ultra-low-leakage memory cell. In implementing this strategy, 0018-9200/$25.00 © 2009 IEEE