An Energy-Efficient ASIC for Wireless Body Sensor Networks in Medical Applications-kmb.pdf

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 1, FEBRUARY 2010

An Energy-Efcient 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-efcient application-specic 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), recongurable 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

exible communication and power-control modes for energy-ef-

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

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.

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-

cient operations. Specically, 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 specically 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 nd 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.

I. I NTRODUCTION

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 gures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identier 10.1109/TBCAS.2009.2031627

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R ECENTLY, researchers are spending great efforts on the

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 1, FEBRUARY 2010

A low-power microcontroller unit (MCU) has been imple-

mented for the ASIC to accomplish the network protocol and the

media-access controller (MAC). The ASIC has communication

ports to control an off-chip ULP half-duplex transceiver which

serves as the primary communication channel of information ex-

changing and networking. A tradeoff strategy between the op-

erating duty cycle and energy efciency [6] has also been im-

plemented for controlling the primary communication channel.

The secondary passive receiver mentioned before can also pro-

vide the function of information exchanging, though the data

rate is lower compared to the primary channel. In addition, a

power-management unit (PMU) provides the different voltage

levels needed, and the PMU is controlled by the MCU to offer

several power modes in accordance with the communication

modes.

This paper is organized as follows. A work-on-demand pro-

tocol aided by a secondary passive communication channel is

proposed in Section II. The ASIC architecture is presented in

Section III, and the circuit implementation details are given in

Section IV. The measurement results from a fabricated ASCI

and a prototype system are given in Section V.

Fig. 2. (a) States. (b) Work state. (c) MCU power of slave nodes.

B. Work and Standby

Two states are dened for the WBSN slave nodes: 1) work

and 2) standby. The two states are shown in Fig. 2. Fig. 2(a)

indicates that the slave node switches between the work state

and the standby state. Fig 2(b) shows the jobs accomplished

in the work state, such as signal sensing, data processing, and

wireless communication. Obviously, the power consumption of

the function blocks in the slave nodes varies with the node states

as shown in Fig. 2(c), in which the power consumption of the

MCU block is depicted. Compared to the work state, the slave

nodes consume much less current.

Conventionally, a low-power timer is utilized for the standby

state. This timer periodically wakes up the WBSN slave nodes.

The sensing nodes work well in this way since in medical ap-

plications, the sensing nodes usually perform signal sensing pe-

riodically. However, for the event-driven stimulating nodes, pe-

riodical toggling between the work state and the standby state

is not energy efcient. Since we are not sure what time the next

drug delivery or stimulating operations will be as requested by

the master node, the stimulating node has to be activated ade-

quately frequently to listen to the master node. There lies the

contradiction between maximizing the energy efciency and

minimizing the response delay for the stimulation nodes. The

wake-up frequency directly affects the duty cycle ratio in that

the slave node is in the work state. Dene the duty cycle ratio as

II. W ORK - ON -D EMAND WBSN

As stated before, a WBSN in medical applications can have

sensing nodes and/or stimulating nodes. In this section, a com-

parison will be made between these two types of slave nodes,

and an energy-efcient WBSN protocol will be proposed with

the feature of work-on-demand.

A. Sensing and Stimulating Nodes

In a WBSN, the sensing nodes and the stimulating nodes

show quite different operation characteristics. A sensing node

usually performs biomedical signal sensing and data transmis-

sion periodically. For example, a glucose detector wakes up

and measures the blood sugar level every 5 min. Since most

biomedical signals have a very low updating rate, the sensing

nodes usually work in a manner of low duty cycle. A stimu-

lating node feeds stimulus back to the human body whenever

needed, such as the instant insulin injection when abnormally

high blood sugar is detected in an automatic insulin pump with

closed-loop control.

A typical scenario of how the sensing nodes, the stimulating

nodes, and the master node operate interactively in a WBSN is

described as follows.

1) The sensing nodes wake up and sense the biomedical sig-

nals periodically.

2) Once the sensing nodes detect any abnormality, an emer-

gency event is reported to the master node immediately.

3) The master node makes the decision accordingly, and

wakes up the corresponding stimulating node if needed.

4) The stimulating node performs medical treatment as de-

manded by the master node.

In this typical scenario, it is clearly seen that sensing nodes are

activated in a periodical way, while the simulating nodes work

in an event-driven manner.

If a small duty cycle ratio is chosen (e.g., 100 ms/1 h), the

stimulating node will have quite low average power consump-

tion if the standby mode is designed well. However, the stim-

ulating node might miss all of the stimulating requests within

the time slot of which lasts for one hour, and this is al-

most unacceptable for medical applications. On the other hand,

if a relatively large (e.g. 100 ms/5 s) is chosen, the risk

of missing stimulating demands from the master node will be

greatly reduced, in the cost of large power consumption, since

the slave node consumes much more power in the work state

than in the standby state.

Hence, the necessity arises for a “work-on-demand” solution

for WBSN slave nodes, especially for the stimulation nodes.

This solution should provide high energy efciency and short

response latency simultaneously.

C. Work-on-Demand With a Secondary Channel

The WBSN master node and slave nodes usually have a bidi-

rectional communication channel to exchange information. Let

us call it the primary channel. Conventionally, at the end of the

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ZHANG et al. : ENERGY-EFFICIENT ASIC FOR WBSN S IN MEDICAL APPLICATIONS

TABLE I

D IFFERENCES B ETWEEN P RIMARY AND S ECONDARY C HANNELS

standby state, the primary channel communication is started,

and the slave nodes listen to the master node.

In this paper, we propose a secondary communication

channel in addition to the primary channel for the WBSN.

The communication is one way, and the master node has a

transmitter, while the slave nodes only have a passive receiver

for this channel. This secondary channel has the following

features:

1) the passive receiver in the slave node does not consume any

current from its own battery; instead, the receiver has an

energy harvesting block to convert the received RF signals

to a dc power supply.

2) the passive receiver in the slave nodes is always ready to

receive any emergency commands from the master node;

3) the transmitter in the master node transmits not only useful

information but also energy to the slave nodes.

With the secondary channel, the master node can wake up the

slave nodes in the standby state at any time if necessary. From

the slave node side, the standby state does not consume any

power (no timer) and the wake-up procedure does not require

any energy (no active listening).

The major differences between the primary channel and the

secondary channel are listed in Table I. It is clear that the sec-

ondary channel needs simplied modules at the receiver end

(slave node).

Fig. 3. Typical scenario of WBSN: sensing and stimulating nodes.

level is detected by the sensing node, the master node should

give warnings and send commands to the stimulating node, then

insulin is delivered by the stimulating node.

The control procedures of the proposed WBSN protocol are

listed as follows.

1) Node I (congured to active standby mode) wakes up peri-

odically to collect biomedical information data and trans-

mits the data to the master node.

2) The master node receives and analyzes the data from node

3) If the master node nds some data abnormal, it needs to

decide the necessary step.

4) The master node transmits emergency command (con-

taining node II’s ID information) through the secondary

channel. Note that node II is congured to passive standby

mode.

5) All of the slave nodes receive the emergency command

including node I, but only node II responds after ID

recognition.

6) Node II wakes up immediately and performs the function

as requested (e.g., driving the insulin pump, giving nerve

stimulus, etc.).

High energy efciency has been achieved for the WBSN de-

scribed before, by utilizing the proposed two standby modes

properly. Also, please note that the real-time work-on-demand

capability has been achieved with the additional secondary pas-

sive channel.

D. Two Standby Modes

With the primary and secondary communication channels in-

tegrated altogether, the WBSN slave nodes can have two modes

for the standby state: 1) the active standby mode and 2) the pas-

sive standby mode. In the active standby mode, the slave nodes

use the primary channel to periodically listen to the commands

from the master node. In the passive standby mode, the slave

nodes only use the secondary channel for passive emergency

listening. Standby mode II has much higher energy efciency

than standby mode II, in the cost of signal receiving sensitivity

and communication distance.

A typical scenario of the proposed WBSN is shown in Fig. 3,

with normal and emergency communications in both channels.

In this scenario, we have an assumption that slave node I is a

sensing node and sensor node II is a stimulating node. With the

proposed architecture, sensor node I is congured to use the

active standby mode, while sensor node II can be congured

to use the passive standby mode in the standby state. Note that

the master node and the two slave sensor nodes in Fig. 3 form a

sensing-coordinator-stimulating structure that is widely used for

WBSNs in medical applications. For example, in blood glucose

monitoring of diabetic patients, whenever abnormal blood sugar

III. ASIC A RCHITECTURE

With the proposed protocol in Section II, a WBSN sensor

node with a hybrid of active/passive RF is introduced to re-

alize the work-on-demand capability as well as high energy

efciency. Utilizing the passive RF receiver for the secondary

channel, the always-on slave sensor nodes can listen passively

to the master node, and can respond to the master node’s re-

quest with a much shorter response time. The architecture of

the sensor node and the ASIC will be presented in this section.

The function block diagram of a proposed WBSN sensor

node is shown in Fig. 4. The sensor node can be divided into

six major function blocks: 1) a digital core for controlling and

processing; 2) a power-management unit; 3) an active bidirec-

tional RF transceiver for data link; 4) a passive RF receiver for

the work-on-demand capability; 5) a state/standby mode control

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IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 4, NO. 1, FEBRUARY 2010

TABLE II

P OWER M ODES IN S TANDBY S TAT E

Fig. 4. Function blocks of a slave sensor node.

B. Power Management

The proposed sensor node is powered by a 3-V battery

power supply. The PMU converts the 3-V power supply into

the voltages levels as needed by other function blocks. Two

programmable linear regulators are integrated in the ASIC for

this function. Specically, the digital core is supplied by a 1.8-V

supply generated by one regulator, and the other analog blocks

used are the 2.5-V analog VDD supplied by another regulator.

Another function of power management is to enable/disable

all of the function blocks (including the linear regulators) ac-

cording to the state and standby mode control and the commands

from the remote master node. The power-mode control logic is

powered directly by the battery. It makes the decision whether

and when the other modules should be switched ON / OFF . There

are only a few ip-ops in this logic circuit that consume power

only when the states change.

In the work state, the power-mode control logic in the PMU

will shut down part of the function blocks if there is no need

to turn them on, according to the presetting stored in the reg-

ister bank or any setup command from the remote master node.

Proper usage of this function provided by the ASIC will greatly

help to improve the system power efciency.

In the standby state, almost all of the function blocks are dis-

abled. For the active standby mode, only the ULP timer with

a low-frequency clock generator is enabled, while in the pas-

sive standby mode, all of the circuit blocks are disabled ex-

cept for the passive RF receiver listening to the master node

power-silently. Table II summarizes the power-mode control for

the two standby modes.

block for energy-efcient operations; and 6) the sensing/stimu-

lating devices for biomedical signal sensing and stimulating.

The main task of this paper is to verify the proposed protocol

with an energy-efcient work-on-demand capability. To accom-

plish this task, an ASIC has been designed and fabricated with

the function blocks enclosed by the bold line in Fig. 4. The dig-

ital core is composed of a main control unit (MCU), a boot-

loader, instruction memory, and data memory. The power-man-

agement unit (PMU) is mainly composed of two low-dropout

linear regulators which generate an analog VDD and a digital

VDD, respectively. The state/standby mode control block con-

tains the standby mode decision logic and an ULP timer for pe-

riodical wakeup. The passive RF receiver provides the function

of RF signal receiving without quiescent current.

The interfaces between the ASIC and the remaining function

blocks are compatible with the common peripheral protocols,

such as SPI,

, general-purpose parallel, etc.

A. Control Flow

The control ow of the slave sensor nodes in the proposed

WBSN is shown in Fig. 4. With the control ow, the slave nodes

can accomplish the sensing-processing-communicating-exe-

cuting ow under supervision of the master node. The remote

master node can also congure the slave nodes’ states and

modes through this control ow.

The primary wireless communication channel for the data

link adopts the half-duplex contention-based protocols. The up-

link (from the slave node to the master node) is for biomed-

ical information data transmission and the downlink (from the

master to the slave) is for sensor-node conguration and stim-

ulating commands. Forward error controlling (FEC) and auto-

matic repeat request (ARQ) are utilized to ensure communica-

tion quality.

In this control ow, the slave nodes can awake up from the

standby state when triggered by two events: 1) a local timeout

signal from the timer in the ASIC and 2) a remote signal from

the master node, corresponding to the two standby modes. The

standby mode of a slave node can be congured by the master

node remotely, and the slave node can accept the wake-up

trigger that matches its standby mode.

IV. C IRCUIT I MPLEMENTATION

The circuit implementation of the ASIC described in Sec-

tion II will be presented in this section.

A. Digital Core

The digital core contains the MCU, bootloader, and multi-

mode transducer interface, as shown in Fig. 6.

An MCU with the basic 8051-compatible instruction set can

be adequate to implement single-hop network protocols suit-

able for WBSN. The control ow in Fig. 5 is implemented in

software. Furthermore, for exibility of multipurpose sensor

nodes’ control ow, the program in the memory ( ,

FLASH, etc.) can be remotely programmable for better control

of different applications. A bootloader module is implemented

to complete the initializing congurations.

The bootloader initializes the MCU status, register le, and

plays the role of memory arbiter. If we want to assemble several

sensor nodes of different functions with the same hardware re-

source, we expect to rewrite the MCU program in the memory.

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Fig. 7. Schematic of the LDO.

Fig. 5. Control ow of slave nodes.

Fig. 8. Block diagram of the passive RF receiver.

adopts a classical self-biased cascade structure to eliminate the

dependence of supply voltage and temperature.

Fig. 6. Digital core functional blocks.

C. Passive RF Receiver

The designed passive RF receiver is mainly composed of an

energy harvesting block and an on-off keying (OOK) demod-

ulator as shown in Fig. 8. To activate the sensor node in the

standby state (specically under the passive standby mode), the

remote master node transmits commands modulated onto an RF

carrier to the sensor node. The energy harvesting block converts

the received RF energy to a dc supply voltage for the demodu-

lator, and uses the recovered energy for signal demodulation.

The energy harvesting block, as shown in Fig. 9(a), converts

part of the incoming passive RF channel signal power to a dc

voltage (VRF) which supplies all of the circuits in the pas-

sive RF module. A multistage rectier is used to convert the

received 915-MHz RF power into a dc power supply stored

on a 300 pF on-chip capacitor. The rectier has a structure of

the Dickson voltage multiplier [16]. This kind of circuit needs

low threshold diodes or metal–oxide semiconductor eld-effect

transistors (MOSFETs). The OOK demodulator mainly con-

sists of a clock and data recovery block (CDR), as shown in

Fig. 9(b). The CDR recovers a digital baseband signal together

with a synchronous clock signal from the received RF signal

with OOK modulation. Sequentially, the identication and com-

mand recognition operation performs digital processing, such as

cyclic redundancy checks (CRC) to valid the received ID code

and the control command. Finally, the passive RF module gener-

ates proper enable/disable control signals corresponding to the

veried incoming command.

The passive RF uses a carrier frequency of the industrial-sci-

entic-medical (ISM) 915-MHz band and has a data rate of

The bootloader can control the memory arbiter to rewrite the in-

struction memory through the active RF communication link or

under the debug mode (wired). Every time the MCU wakes up

from sleep mode back into work, its key status must be restored,

guaranteed by CRC-8 verication.

The implemented digital core supports multimode peripheral

interface including

, SPI, and general-purpose parallel ports

for exibility [17].

B. Power-Management Unit

The PMU in this ASIC is mainly composed of two linear

regulators and the power-mode control logic. The low-dropout

regulators (LDOs) regulate the battery voltage 3 V down to de-

sired output voltages with trimming capability. Fig. 7 shows the

LDO circuitry design is similar to that in [9]. It is composed of

an error amplier (M1 M8); a unit-gain buffer (M9 M12)

[10]; a PMOS pass device; a feedback network (R1, R2); and

an off-chip loading capacitance . An Ahuja compensation

method [11] rather than Miller compensation is used. The Ahuja

compensation exhibits excellent phase margin across all loads as

well as higher power-supply ripple rejection ratio (PSRR) than

classical Miller compensation.

An integrated bandgap reference voltage generator provides

reference voltages and bias currents for the other circuit. It

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