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Paper presentation Ultra-Portable Devices Paper: 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.

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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 1 Paper Presentation - Ultra Portable Devices

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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

2010-05-31 2 Paper Presentation - Ultra Portable Devices

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System diagram of typical WBSN applications

2010-05-31 3 Paper Presentation - Ultra Portable Devices

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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

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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

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Typical scenario of WBSN

  • peration
  • 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.

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(a) States (b) work state (c) MCU power of slave nodes

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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

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Low duty cycle: minimizing energy, maximizing response delay or missing the stimulating requests High duty cycle: maximizing energy, minimizing response delay

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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

2010-05-31 Paper Presentation - Ultra Portable Devices 9

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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.

2010-05-31 Paper Presentation - Ultra Portable Devices 10

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Differences between primary and secondary channels

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Typical scenario of WBSN: sensing and stimulating nodes

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Function blocks of a slave sensor node

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Control flow of slave nodes

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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

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Power modes in standby state

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Circuit implementation:

  • A. Digital core functional blocks

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Circuit implementation:

  • B. Power management unit, Schematic of the LDO

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M1-M8: error amplifier M9-M12: unit gain buffer R1, R2: feedback network

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Circuit implementation:

  • C. Block diagram of the passive RF receiver

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Passive RF schematic:

(a) Energy recovery for the dc supply and (b) clock data recovery (CDR)

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Circuit implementation:

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

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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

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Circuit implementation:

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

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Used for th ULP timer to tune the osc. freq. Simulated power consumption: 4 µW in 0.18 µ CMOS

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ASIC on the testing board and die photo

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0.18µm standard CMOS technology Die area: 2.0mm x 1.5 mm

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WBSN prototype setup

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Waveforms of the WBSN sensor node:

(a) 500 ms/div. (b) 250 µs/div.

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Passive RF receiver performance

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SENSOR NODE POWER WITH THE ASIC, UNDER DIFFERENT STATES AND MODES

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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

2010-05-31 28 Paper Presentation - Ultra Portable Devices