Energy Efficient Design and Energy Harvesting forEnergy Autonomous Systems
Makoto Takamiya

Requirements for IoT sensor nodes, wearable healthcare
devices, and implanted medical devices are the wearingunconsciousness and the maintenance-free operation as shownin Fig.1. To enable the wearing-unconsciousness, mechanically
flexible or small-size devices with the wireless connection arerequired. To enable the maintenance-free operation, energy
autonomous devices are required. The energy autonomy isachieved by both the energy efficient operation and the energy
harvesting. In this paper, energy autonomous systems with theenergy efficient design and the energy harvesting are shown.
II. ENERGY EFFICIENT DESIGN
The energy efficient operation is achieved by a nearthreshold operation and a temporal-spatial fine-grained control.
Fig. 2 shows the simulated power supply voltage (VDD)
dependence of power, delay, and energy. Compared with thenominal operation at VDD = 1.2V, the energy of the nearthreshold operation at VDD = 0.3V is reduced to one-tenth ofthat at VDD = 1.2V. Fig. 3 shows the temporal-spatial finegrained control [1]. In the conventional design, the clock
frequency (fCLK), VDD, and the threshold voltage (VTH) oftransistors are common within a chip. In contrast, in the state-

10-2

10-3

10Delay (a.u.)-1Power (a.u.)

I. INTRODUCTION

010

104

10

3

102

10

1

100-4

10-5

10

1.0
0.9
1x
10
0.8
0.7
0.6
0.5
NominalVDD (1.2V)
0.4
0.3
0.2
0.1
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

Power supply voltage (VDD) (V)

Fig. 2. Power supply voltage dependence of power, delay, and energy.

Future chip

Temporal scale of control

Abstract— Energy autonomy enabled by the energy efficientdesign and the energy harvesting is the one of the top
requirements for maintenance-free IoT sensor nodes andwearable/implanted devices. In this paper, energy efficient ultralow voltage (< 0.5V) circuits are shown. Energy autonomous
wearable healthcare devices using the flexible, large-area, anddistributed organic electronics are also shown.

Energy (=Power x Delay) (a.u.)

University of Tokyo
Tokyo, JapannsDomain1
fCLK1, VDD1, VTH1

123

fCLK3, VDD3, VTH34nPs

Past chipSingle domain

StaticfCLKn, VDDn, VTHn

Temporal-spatial
fine-grained control
for energy efficiency

CommonfCLK, VDD, VTH
110100

Number of domains within a chipfCLK: Clock frequency, VDD: Supply voltage, VTH: Threshold voltage

Wearing-unconscious
NikeSonyFig. 3. Temporal-spatial fine-grained control.

(1) Flexible / small-sizeof-the-art design, many different fCLK’s, VDD’s, and VTH’s areused within a chip and they are dynamically changed tominimize the energy. Such temporal-spatial fine-grained
control includes the dynamic voltage scaling [2], the dynamicfrequency scaling [3], the power gating, the clock gating, thebody biasing, the local gate overdrive [4], and the quick wakeup circuits [5].

(2) Wireless

Maintenance-free(3) Energy autonomy(Low energy + Energy harvesting)Fig. 1. Requirements for wearable/implanted devices.

978-1-4799-6275-4/15/$31.00 ©2015 IEEE

Fig. 4 shows a block diagram of extremely low power 0.5V
SoC for IoT sensor nodes. The voltage obtained from theenergy harvester is regulated by the power management
circuits [6-7] and many different VDD’s are given to A/DAll flexible components

Extremely low power 0.5V SoC

Patient

Temperature detector

Power management(15mm x 5mm)

TEG

0.6V

0.5VSensor

A/D
converter

0.5V 0.6V

Logic

0.5V 0.5V

0.2V

RFSRAM

Organic circuits

TEG: Thermoelectric generator(under speaker)

Amorphous Si
solar cells

Fig. 4. Block diagram of extremely low power 0.5V SoC for IoT sensor nodes.

Piezoelectric speaker

SRAM

5 mm

CPU
1CPU
05 mm

5 mm x 5 mm

Number ofTransistors

31 M (Logic䠖䠖16 M,SRAM䠖15 M)

Logic cores

CPU32 bit

SIMD

4 bit, 320 PE

CPU

Inst.

1 MbitData

256 kbitInst.

512 kbitImage

128 kbit x 4Work

64 Byte x
320 (2port)

SIMD

SRAM

SIMD

40 nm CMOS

Die size

SRAM

PLL

Buck

Buck

ADC

Frequency

10 MHzSupplyvoltages

VLOGIC

0.5 V

VCLK

0.6 V

VSRAM1

0.8 V (1 port)

VSRAM2

0.6 V (2 port)

Power(11cm x 13cm)

for fever alarm(10cm x 5cm)

Technology

18 cm

Solar cell

30 cmFig. 6. Fever alarm armband [10].

VDD

Solarcells

Activevoltagelimiter

Flexible
Temperature EN
piezoelectricdetector
speaker

Organic complementary circuits

9 mW

SIMD
559 GOPS/W
performance (5.73 mW@10 MHz)
Max. 563 GOPS/W(4.26 mW@7.5 MHz)

Fig. 7. Block diagram of fever alarm armband [10].

voltage limiter regulates VDD. When the measured temperature
is higher than the preset threshold temperature, a ring oscillator
starts oscillation and the speaker makes a sound.

Fig. 5. 0.5V image processor [3].

converter, logic circuits, SRAM, and RF circuits for the
temporal-spatial fine-grained control to minimize the energy.
Fig. 5 shows an example of 0.5V SoC [3]. The 0.5V image
processor with 563 GOPS/W SIMD and 32bit CPU includes anall digital PLL (ADPLL), an A/D converter, and buckconverters.

ACKNOWLEDGMENT
This work is supported in part by JST/ERATO.
REFERENCES[1]

III. ENERGY AUTONOMOUS WEARABLE FLEXIBLE DEVICES
The organic electronics enables flexible, large-area, anddistributed sensor and/or actuator array and is suitable for thewearing-unconscious devices. Several energy autonomous
wearable healthcare devices using the organic electronics havebeen proposed [8-9].
Fig. 6 shows a photograph of a fever alarm armband (FAA)[10] integrating fully flexible solar cells, a piezoelectric
speaker, a temperature detector, and 12V organic
complementary FET circuits. FAA is a flexible energy
autonomous healthcare device with the wireless interface. FAA
is looped around an upper arm of a patient in a hospital room,and the temperature detector monitors the underarm
temperature of the patient. 220-Pm thickness amorphoussilicon solar cells attached outside of the upper arm generate
the power. When high fever is detected, a 52-Pm thickness
piezoelectric speaker with polyvinylidene difluoride (PVDF)
makes a sound to alarm a nurse. Organic circuits and thetemperature detector are fabricated on a 50-Pm thickness
flexible polyimide film, and the solar cells and the speaker areattached on it. Fig. 7 shows a block diagram of FAA. An active

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