IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 1, FEBRUARY 2010 11 An Energy-Efficient ASIC for Wireless Body Sensor Networks in Medical Applications Xiaoyu Zhang, Hanjun Jiang, Member, IEEE, Lingwei Zhang, Chun Zhang, Zhihua Wang, Senior Member, IEEE, and Xinkai Chen Abstract���An energy-efficient application-specific integrated circuit (ASIC) featured with a work-on-demand protocol is de- signed for wireless body sensor networks (WBSNs) in medical applications. Dedicated for ultra-low-power wireless sensor nodes, the ASIC consists of a low-power microcontroller unit (MCU), a power-management unit (PMU), reconfigurable sensor interfaces, communication ports controlling a wireless transceiver, and an integrated passive radio-frequency (RF) receiver with energy har- vesting ability. The MCU, together with the PMU, provides quite flexible communication and power-control modes for energy-effi- cient operations. The always-on passive RF receiver with an RF energy harvesting block offers the sensor nodes the capability of work-on-demand with zero standby power. Fabricated in standard 0.18- m complementary metal���oxide semiconductor technology, the ASIC occupies a die area of 2 mm 2.5 mm. A wireless body sensor network sensor-node prototype using this ASIC only consumes 10-nA current under the passive standby mode, and 10 A under the active standby mode, when supplied by a 3-V battery. Index Terms���Energy harvesting, passive RF, wireless body sensor network (WBSN), work-on-demand. I. INTRODUCTION R ECENTLY, researchers are spending great efforts on the wireless body sensor networks (WBSNs) for medical ap- plications, such as vital sign monitoring, the diagnose assis- tant, and the drug delivery [1]���[3]. Fig. 1 shows some typical applications in WBSN. In these applications, rather than the peer-to-peer self-organized network topologies, the single-hop star network topology and the master-slave protocol are com- monly adopted to lower the system complexity and power con- sumption as well [1], [4]. A typical WBSN is usually composed of a portable device which serves as the master node for cen- tral control, and a number of miniaturized sensor nodes placed Manuscript received March 13, 2009 revised July 06, 2009. First published November 03, 2009 current version published January 27, 2010. This work was supported by the National High Technology Research and Development Program of China (863 Program) (No. 2008AA010707). This paper was rec- ommended by Associate Editor S. Hu. X. Zhang is with the Department of Electronic Engineering, Tsinghua Univer- sity, Beijing 100084, China (e-mail: zhangxiaoyu00@mails.tsinghua.edu.cn). H. Jiang, L. Zhang, C. Zhang, and Z. Wang are with the Institute of Microelectronics, Tsinghua University, Beijing 100084, China (e-mail: jianghanjun@tsinghua.edu.cn zlw03@mails.tsinghua.edu.cn zhangchun@ts- inghua.edu.cn zhihua@tsinghua.edu.cn). X. Chen is with Ecore Technologies Ltd., Beijing 100086, China (e-mail: chenxk@ecore-tech.com). 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/TBCAS.2009.2031627 Fig. 1. System diagram of typical WBSN applications. around, on, or inside the human bodies that act as the slave nodes. Compared to the master node, the slave nodes have more stringent constraints in terms of power consumption and size limitation. And this work mainly focuses on the slave sensor nodes in the WBSNs. Typical WBSN slave sensor nodes can be used for biomedical information acquisition, signal preprocessing, data storage, and wireless transmission (sometimes direct transmission without any preprocessing). This type of slave sensor node is called the sensing node. In addition, the function of sensor nodes can be expanded to medical treatments, such as drug delivery and nerve stimulating [5], and this type of slave sensor node is called the stimulating node. One difference between the two types of nodes is that the functions of a sensing node are usually period- ically performed, while the functions of a stimulating node can be either periodical or event driven. A study has been made on these two types of WBSN nodes, and a network protocol has been proposed and implemented which meets the requirements of both, targeting the power-ef- ficient operations. Specifically, the implemented ASIC has two standby modes. In the active standby mode, only an ultra-low- power (ULP) timer with a low-frequency clock generator is ac- tive, and it periodically power ups the sensor node. In the pas- sive standby mode, the whole sensor node is power silent, and a secondary passive RF receiver works as the supervisor circuit. The specifically designed passive RF receiver can harvest en- ergy from the RF signals in the space (transmitted by the master node which is not power critical), and hence, the passive standby mode consumes zero power ideally. The active standby mode can be used for the sensing and stimulating nodes. As a con- trast, the passive standby mode can find its perfect use for the stimulating nodes, since the event-driven stimulating nodes can be woken up on demand without any response latency, while consuming zero power. 1932-4545/$26.00 �� 2009 IEEE