xiaoyu zhang, et al. an energy-efficient asic for wireless body

Paper presentation – Ultra-Portable Devices

Paper:

Presented by:

Xiaoyu Zhang, et al.

An Energy-Efficient ASIC for Wireless Body Sensor

Networks in Medical Applications

IEEE transactions on biomedical circuits and systems,

vol 4, no. 1, pp. 11-18, Feb. 2010.

Reza Meraji

2010-05-31 1Paper Presentation - Ultra Portable Devices

Outline

• Introduction of wireless body sensor network (WBSN)

• Work on demand solution for power efficiency

– Sensing and stimulating nodes

– Work and standby modes

– Introducing a secondary channel

– Active and passive standby nodes

• ASIC architecture

• Circuit implementation

• Measurement results

• Summary


System diagram of typical WBSN

applications


Typical WBSN slave sensor nodes

• Sensing node:

– Biomedical information acquisition, signal processing, data storage,

wireless transmission (sometimes direct transmission without any

processing)

– Functions of sensing node are usually periodically performed

• Stimulating node:

– Medical treatment, drug delivery, nerve stimulating, etc.

– Functions of stimulating node can be either periodical or event driven

2010-05-31 Paper Presentation - Ultra Portable Devices 4

Proposed standby modes for

WBSN

• Active standby mode (for sensing and stimulating nodes):

– Only an ultra low power (ULP) timer with a low-frequency clock

generator are active

– It periodically powers up the sensor node

• Passive standby mode (for the stimulating nodes):

– The whole sensor node is power silent

– A secondary passive RF circuit works as the supervisor circuit

– The passive RF receiver can harvest energy from the RF signals

transmitted by the master node

– The passive standby node consumes zero power ideally


Typical scenario of WBSN

operation

• 1) The sensing nodes wake up and sense the biomedical

signals periodically.

• 2) Once the sensing nodes detect any abnormality, an

emergency 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

demanded by the master node.


(a) States (b) work state (c) MCU

power of slave nodes


Work and standby: energy

efficiency

• Periodical toggling between work and standby modes:

– Suitable approach for the sensing nodes since these nodes should

sense/process/transmit data periodically

– Not energy efficient for the event-driven stimulating nodes


Low duty cycle: minimizing energy, maximizing response delay or

missing the stimulating requests

High duty cycle: maximizing energy, minimizing response delay

Work-on-demand solution with a

secondary channel

• Primary channel:

– Bidirectional communication channel to exchange information

• Secondary channel:

– Communication channel is one-way

– Master node has a transmitter and slave nodes only have a passive

receiver for this channel


Features of the secondary channel

• 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.


Differences between primary and

secondary channels


Typical scenario of WBSN:

sensing and stimulating nodes


Function blocks of a slave sensor

node


Control flow of slave nodes


Power management

• Sensor node ASIC is powered by a 3-V battery power supply

• Two linear regolators are integrated to convert 3-V power

supply into the other voltage levels

• Digital core is powered by a 1.8-V supply generated by the

regolator

• Analog blocks are powered by a 2.5-V supply from the other

regolator


Power modes in standby state


Circuit implementation: A. Digital core functional blocks


Circuit implementation:B. Power management unit, Schematic of the LDO


M1-M8: error amplifier

M9-M12: unit gain buffer

R1, R2: feedback network

Circuit implementation:C. Block diagram of the passive RF receiver


Passive RF schematic:(a) Energy recovery for the dc supply and

(b) clock data recovery (CDR)


Circuit implementation:D. Low power clock generation: (a) 24-MHz clock generation


Used for the digital core

Capacitor C0 and C1 are

charged and discharghed

by turns

By merging VC0 and VC1

a saw-tooth wave is

generated at node N0 and

compared with a reference

voltage

A second comparator U1

Resets the oscillator if the

Frequency is too high

Simulated power consumption:

625 µW in 0.18 µ CMOS

Circuit implementation:D. Low power clock generation: (b) 20-kHz clock generation


Used for th ULP timer

to tune the osc. freq.

Simulated power consumption:

4 µW in 0.18 µ CMOS

ASIC on the testing board and die

photo


0.18µm standard CMOS technology

Die area: 2.0mm x 1.5 mm

WBSN prototype setup


Waveforms of the WBSN sensor node:

(a) 500 ms/div. (b) 250 µs/div.


Passive RF receiver performance


SENSOR NODE POWER WITH THE ASIC, UNDER

DIFFERENT STATES AND MODES


Summary

• The standby power issue and the response latency in the

WBSN have been inspected in this work

• an energy-efficient protocol with work-on-demand has been

proposed for WBSN

• Compared to the conventional structure, the proposed

WBSN slave sensor node has a passive secondary wireless

receiver


xiaoyu zhang, et al. an energy-efficient asic for wireless body

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