TandemStack

A Flexible and Customizable Sensor Node Platform for Low Power Applications

Oliver Stecklina

, Dieter Genschow

and Christian Goltz

IHP GmbH, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany

BTU Cottbus, Konrad-Wachsmann-Allee 1, 03046 Cottbus, Germany

Keywords:

Motes, Sensor Node, Wireless Sensor Networks, Low Power, Low Cost, Micro-controller, Transceiver.

Abstract:

Wireless sensor nodes are becoming more and more considered for a wide variety of application scenarios. But

by going into real world scenarios requirements becomes more complex and must be covered more accurate.

Furthermore, these requirements must be met by a development process that is driven by the factors of cost and

time. Reusing components is a promising way to make such a development process more efﬁcient. We present

a ﬂexible and customizable sensor node platform which follows the objective to assemble as few functional

units as possible on a single Printed Circuit Board (PCB). The PCBs are connected by a standardized board-

to-board connector, which makes a free ordering and mixture of different modules possible. We will show that

by using a standardized Mote Component Interconnect (MCI) an extension as well as an adaption of a sensor

node to new scenarios becomes feasible by adding or replacing single platform components. The presented

sensor node platform allows a fast and inexpensive development process as it is necessary for current and

upcoming real word applications.

1 INTRODUCTION

Wireless Sensor Network (WSN)s become more and

more common in a wide variety of application sce-

narios. They are used for controlling industrial in-

frastructures, urban and environmental monitoring as

well as home-care applications. Along with their

growing use cases a ﬂexible sensor node platform is

needed to meet upcoming and changing requirements

in a fast and cost-efﬁcient way.

The ﬂexibility of the ubiquitous Personal Com-

puter (PC) architecture with standardized buses is an

important factor for its triumphal procession. A mod-

ern PC can be easily and cost-efﬁciently extended or

adapted to new or changing requirements by replac-

ing or adding expansion cards. Furthermore, stan-

dardized buses open the opportunity to setup systems

with components from many different manufacturers.

We state that a standardized interface for mote com-

ponents can reduce the development costs and time

for mote’s hardware and software signiﬁcantly. In this

paper we present a ﬂexible and customizable mote

platform that follows the objective to combine only

tightly coupled components on a single PCB. The dif-

ferent modules will be connected by a uniform Mote

Component Interconnect (MCI), which allows a new

freedom in combination. We will introduce some re-

strictions to ensure the interoperability of modules

and present concepts for routing buses or point-to-

point links over a uniform interface.

In the next section we will introduce a set of

more or less well-known commercially available sen-

sor nodes. All these motes are designed for low

power WSNs without real application backgrounds.

But their inﬂexible architectures limit their applica-

tion ﬁelds signiﬁcantly. In the following we present

our TandemStack platform. We introduce the MCI,

which allows a free ordering of modules. In section

5 we compare our design with a selection of motes of

section 2. Finally we describe three application sce-

narios where our TandemStack is already used. After

that we will conclude the paper with a short outlook.

2 MOTE PLATFORMS

We can identify three different types of low power

motes. The ﬁrst type uses a single PCB and is

equipped with the basic mote components: Micro

Controller Unit (MCU), transceiver, storage, sensors

and power supply. The second type offers alterna-

tive components on a single PCB and third one uses a

Stecklina O., Genschow D. and Goltz C..

TandemStack - A Flexible and Customizable Sensor Node Platform for Low Power Applications.

DOI: 10.5220/0003833200650072

In Proceedings of the 1st International Conference on Sensor Networks (SENSORNETS-2012), pages 65-72

ISBN: 978-989-8565-01-3

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

multi layer design, which makes replacement of com-

ponents feasible. A wide varity of all these types of

motes are available.

Single PCB designs like the MicaMote platforms,

TmoteSky (Moteiv, 2006), Telcos (Polastre et al.,

2005) or Sun SPOT (Labs, 2010) are widely used.

Mica2Mote (Horton et al., 2002) and Mica2Dot were

developedat Berkeley and are commercially available

from Crossbow Technology (Crossbow, 2011). Both

modules use an ATMega128L MCU combined with

a CC1000 transceiver. The power supply is a battery

pack or coin cell that can drive the modules a cou-

ple of days in active mode. The MicaDot platform

was developed in 2002 and its MCU and transceiver

are a few years older. Although these types of de-

vices are not rapidly changing like high performance

CPUs, their features and electrical characteristics be-

comes more and more obsolete. The TmoteSky, Tel-

cos and SunSpot designs are very similar. All of

these modules are platforms that can only be extended

via an expansion connector. The TmoteSky uses an

MSP430 MCU and the SunSpot is equipped with a

32-bit ARM-based MCU. While the MSP430 is a

16-bit ultra low power device with outstanding run-

time capabilities, the ARM device has a powerful 32-

bit processing core with a Memory Protection Unit

(MPU). But due to the ﬁxed platform design both

CPUs must be used with the given transceivers and

power supplies. A ﬂexible combination or MCU up-

grade is not possible.

A mote design with alternative devices on a single

PCB is offered by our IHPNode. It was designed as a

mote for a Body Area Network (BAN) for ﬁreﬁghters

(Piotrowski et al., 2010). In contrast to other motes

it is equipped with a modern MSP430 MCU and has

three IEEE 802.15.4 transceivers, a subgigahertz and

two 2.4GHz modules. Furthermore, two types of local

storage, an expansion header and a Universal Serial

Bus (USB) interface are available. All these compo-

nents make the IHPNode very ﬂexible and useful for a

variety of applications. The drawback of this solution

is that an adaption to other radio bands, storage tech-

nologies or a computing power upgrade still require a

mote redesign.

A ﬂexible plug and play system is available by

Coalsenses iSense modules (Coalesenses, 2011). The

iSense core module is based on a Jennic wireless con-

troller. It combines a IEEE 802.15.4 compliant radio

with a 32-bit microcontroller. The core module can

be combined with a number of different modules. A

similar design is used by the Flex Mini Kit (Evidence,

2010) and the building block-approach (Dutta et al.,

2008). But the ﬂexibility of these motes are limited

by their static core module design. A modular and

ﬂexible design in a consequent manner as presented

by our approach is not given.

The Tyndall25 Mote is a ﬂexible mote 25 mm

cube design developed by Tyndall National Institute

(Bellis et al., 2005). The cube design splits a mote

into a series of layers with an equal dimension and

interface. A node is built up from an Field pro-

grammable Gate Array (FPGA) module, which op-

erates as core processing unit. This module can be

combined with sensors, power supply and communi-

cation modules. Every type of module uses a com-

mon interface connector. The FPGA and the commu-

nication module are stack modules with connectorson

top and bottom side. Overlapping connector pins on

these modules are connected to allow a communica-

tion through the cube and between non-adjacent mod-

ules. The compact modular wireless sensor platform

described by the author of (Benbasat and Paradiso,

2005) uses also a module stack design with a uniform

electrical interconnect with dedicated lines. It deﬁnes

various modules, so called panes, with an individual

use, which can be combined to compact sensor node.

But its modularity is limited by its predeﬁned com-

bination schemes. A highly ﬂexible approach is pre-

sented by the power-aware microsensor architecture

(Schott et al., 2005). It is a stackable design with a

core module bus that is very close to the introduced

approach in this paper. But its module compatibility

is implemented by an additional switching ICs, which

increases module size and power consumption. A

similar design is presented by (Lymberopoulos et al.,

2007). It uses a CPLD-based bus controller for con-

necting modules.

The available hardware designs of motes mostly

use a static conﬁguration. A replacement of a main

component like MCU, transceiver or storage is in

most cases not possible. The presented ﬂexible motes

are built around a uniform layer interface uses a

bus controller, which increases cost, size and node’s

power consumption. A ﬂexible ultra low power archi-

tecture with a uniform Mote Component Interconnect

(MCI) is not addressed by any of these approaches.

3 The TandemStack

As presented in section 2 most available motes use

a single PCB design. In contrast, our platform ap-

proach follows the objective to assemble as few com-

ponents as needed for one functional unit on a sin-

gle layer or PCB. Functional units like radio, pro-

cessing core, storage or power supply are separated

on different layers. All these PCBs will be con-

nected by our predeﬁned Mote Component Intercon-

SENSORNETS 2012 - International Conference on Sensor Networks

nect (MCI). This approach provides the opportunity

to easily adapt the mote to customized requirements

by designing a new module and combining this with

already available modules.

A wireless node consists of at least an MCU, a

power source and a wireless communication inter-

face. Although all these components are necessary for

a mote a TandemStack will be assembled on at least

three separate modules. A TandemStack mote con-

sists of three types of modules being bottom, stack

and top modules. One module will usually be assem-

bled on a single PCB, although this is not a restriction

as long as the module acts as one functional unit. An

overview of already implemented TandemStack mod-

ules is given in table 1.

Each TandemStack mote has at least one bottom

module. This module has one (basic version) or two

(extended version) MCI headers on its top side. A

variable number of stack modules can be plugged

onto one bottom module. Each stack module has one

or two headers on its top side and the according recep-

tacles on its bottom side. A stack can be terminated by

a top module, which only has one or two receptacles

on its bottom side. The second MCI connector was

designed for more complex motes and can be placed

on each layer. The MCI pinning is strictly predeﬁned,

which allows a free ordering of stack modules. Fur-

thermore a mixture of single and dual connector mod-

ules in a single stack is possible with certain restric-

tions. Figure 1 shows the TandemStack development

stack with a power supply and debugging module, a

processing module as well as a radio module.

Figure 1: TandemStack development board with a power

and debugging bottom module, a MSP430-based process-

ing and a radio stack module in comparison with an one

cent coin.

3.1 Conﬁguration and Design

Restrictions

In order to achieve interoperability between the mod-

ules the design of a module must follow certain con-

ﬁguration premises. We deﬁned restrictions for the

connector’s pinning, routing and placement as well as

a minimum of required features on a module.

Although Figure 2 shows a recommended outline,

this is not a restriction. We think a predeﬁned board

dimension will reduce mote’s ﬂexibility in a signiﬁ-

cant manner. A designer should have the freedom to

adapt the node to the application requirements instead

of outline restrictions. However, for the sake of in-

teroperability modules must follow predeﬁned place-

ments and pinning for the board-to-board connectors

and position of mounting holes. On a stack module all

header and receptacle pins must be connected (an ex-

ception are user deﬁned pins and uni-directional pins

like the JTAG chain). Furthermore the height of top

side components is limited to 6mm and on the bottom

side it is limited to 1.8mm. These limits are necessary

to ensure that all modules can be plugged together.

On top modules the top side height and on bottom

modules the bottom side height of components is not

restricted.

Figure 2: Dimension and placement of holes for the three

different modules formats. Boards are dimensioned from

the centre. Origin is the lower left corner.

Every TandemStack must have a bottom layer

module. It has to source 3.3V and 5.0V supply. Fur-

thermore, pullup resistors for e.g. I

C and reset lines

have to be placed on a bottom module. Stack and

top modules must not source 3.3V and 5.0V, but may

source an additional raw voltage. This way it is i.e.

possible to build a solar panel power supply as a top

module.

External connectors that provide communication

interfaces to the stack should be placed on the bottom

module. Buttons may also be placed there. Although

top modules are a possible place for buttons and ex-

ternal connectors too, the bottom module should be

preferred, because a bottom module is required in a

stack conﬁguration, while a top module is optional.

TandemStack - A Flexible and Customizable Sensor Node Platform for Low Power Applications

Table 1: List of already implemented TandemStack modules.

Class Type Device Dimension Connectors

Power Supply Bottom Thermo Electric Generator (TEG) ∅ 40mm X1

Power Supply Bottom Battery supply ∅ 40mm X1

Power Supply Bottom USB supply 40mm x 50mm X1, X2

MCU Stack MSP430F161x ∅ 40mm X1

MCU Stack MSP430F5438A 40mm x 50mm X1, X2

MCU Stack Sparc V8 based LEON-2 40mm x 50mm X1, X2

Transceiver Stack 868MHz CC1101 radio ∅ 40mm X1

Transceiver Stack 868MHz CC1190 ampliﬁer ∅ 40mm X1

Transceiver Top Bluetooth Blue-SP ∅ 40mm X1

3.2 Mote Component

Interconnect (MCI)

In contrast to other conﬁgurable motes the Tandem-

Stack uses a strictly deﬁned Mote Component In-

terconnect (MCI). That means each module must

place its board-to-board connectors on deﬁned posi-

tions and must use a predeﬁned pinning. Our MCI

supports a number of common mote interfaces such

as GPIOs, buses, interrupt and analog lines as well as

a common programming interface.

As mentioned before one MCI connector is op-

tional (X2) and one is compulsory (X1). The op-

tional connector X2 will be used only on layers with

a larger footprint. Table 2 shows the deﬁned pining

of connector X1. Connector X1 includes all com-

mon peripheral interfaces of a typical mote. Connec-

tor X2 is an extension of X1 and provides more ana-

log, GPIO, event, power and user deﬁnable lines. Fur-

thermore, for specialized applications two additional

power lines Vraw1 and Vraw2 and four counter lines

are available on connector X2.

We use two 50-pin connectors from Hirose, which

make a high-density of components possible. It has

a 0.5 mm pitch and is available in a range of heights

from 5 to 8mm, which allows a tailoring of the board-

to-board gap for a better matching of a required stack

height. Its high pin density enables the use of highly

integrated MCUs while maintaining a good mechani-

cal stability and ruggedness.

3.2.1 Signal Routing

For a free ordering of stack modules all common MCI

pins must be routed between bottom and top board-

to-board connector. It is permitted to route pins via

a module’s Integrated Circuit (IC), where an IC must

be active forwarder between both connectors. We call

this conﬁguration an active routing. In a passive rout-

ing conﬁguration an IC is only connected to a static

line between both connectors. Furthermore, an ac-

Table 2: Pining of 50-pin MCI connector X1. The connec-

tor must be available on all layers and has s strictly deﬁned

pinning with all common mote peripheral interfaces.

Pin Name Type Pin Name Type

1 5V0 Power 26 GPIO1 GPIO

2 5V0 Power 27 GPIO2 GPIO

3 3V3 Power 28 GPIO3 GPIO

4 3V3 Power 29 GPIO4 GPIO

5 GND Power 30 GPIO5 GPIO

6 TEST JTAG 31 GPIO6 GPIO

7 TCK JTAG 32 GPIO7 GPIO

8 TMS JTAG 33 AD0 Analog

9 TDO JTAG 34 AD1 Analog

10 TDI JTAG 35 AD2 Analog

11 Reset Reset 36 AD3 Analog

12 MCLK Clock 37 IRQ0 Event

13 MISO SPI 30 IRQ1 Event

14 MOSI SPI 31 IRQ2 Event

15 CLK SPI 32 IRQ3 Event

16 CS SPI 41 UDEF0 Udef

17 RX UART 42 UDEF1 Udef

18 TX UART 43 UDEF2 Udef

19 SCL I2C 44 UDEF3 Udef

20 SDA I2C 45 UDEF4 Udef

21 D+ USB 46 UDEF5 Udef

22 D- USB 47 UDEF6 Udef

23 GND Power 48 UDEF7 Udef

24 GND Power 49 GND Power

25 GPIO0 GPIO 50 GND Power

tive routing makes a bus interruption or an isolation

of stack modules feasible. Interrupting buses may be

useful to operate with peripheral devices with differ-

ent bus parameters, e.g. clock speed or data width.

While using passive routing all components share a

bus and must operate with same parameters as well

as all bus members can see and inﬂuence transmitted

data. For safety, dependability and security aspects an

isolation of components can be useful. An active rout-

ing conﬁguration is also useful to connect a plurality

SENSORNETS 2012 - International Conference on Sensor Networks

of components to a peer-to-peer link, like UART or

USB.

3.2.2 Signal Types

We classiﬁed the signals used on motes in 13 differ-

ent types. Most of these types are supported by all

commercially available MCUs. Classifying mote sig-

nals will improve the module’s interoperability. In the

following we will introduce all 13 types in detail.

Power is used for a module’s power supply. It

includes 5.0V, 3.3V and ground lines. The Hirose

board-to-board connector limits the current on single

pins to 300mA. For supporting higher current mod-

ules a multiple number of power pins are deﬁned. In

addition to the basic supply voltage two Vraw lines

are deﬁned. These lines can be used with a user spe-

ciﬁc voltage.

GPIO lines are general digital input/output lines

with no special properties other than being able to ac-

cept 5V even when operating on 3.3V. This restriction

is true for any non-power signal.

USB lines can be used to connect ICs and/or ex-

ternal connectors via USB. Although USB requires

a point-to-point connection and only one component

can be connected to the lines D+ and D-, a USB net-

work can be deployed over a TandemStack by using

USB hubs, as shown in Figure 3. USB is strictly dif-

ferenced in master and slave, so that or a free module

ordering a slave must be connected to the bottom side

receptacle and a master to the top side header.

DEVICE

HUB

bottom

top

Figure 3: Routing of USB lines on a stack module by using

a hub in an active routing conﬁguration.

Event lines are a special type of GPIO lines.

These lines are connected to IC pins with interrupt

support for asynchronous event handling. It can also

be used as a digital GPIO, if the available number of

GPIOs is not sufﬁcient.

Analog lines are GPIO lines, which are connected

to an A/D converter input. In contrast to GPIO lines

the signal level of analog lines is not deﬁned and can

be any value between zero and one of the stack’s sup-

ply voltage. A use as GPIO line is not recommended

to ensure compatibility.

Clock line can be used to deploy an IC clock over

a multiple number of modules. The line is driven by

a single module, normally the MCU module, and can

be used by a multiple number of ICs with external

clock input.

Reset line is used as a low active system reset.

The signal is fed to all modules and can be used to

synchronize stack ICs. A reset network can be built

up by using active routing.

JTAG is the stack programming interface. It sup-

ports device chaining, which makes the number of re-

quired lines independent from the number of devices.

Figure 4 shows the Joint Test Action Group (JTAG)

chain in the TandemStack. An external connector

must be placed on the bottom module. On each stack

module the Test Data In (TDI) bottom connector pin

is connected with the IC’s TDI, while its Test Data

Out (TDO) is forwarded to the top connector. On a

top module the JTAG chain must be closed by con-

necting IC’s TDO with the bottom connector TDO

pin. By the fact that a top module is optional each

stack module may have a conﬁgurable short.

UDef can be used for coupling two familiar mod-

ules. Although a strict limitation for the module’s di-

mension is not given a module may need more than

one layer to assemble all required components. In this

case a module can be easily spread over more than

one layer by using the user deﬁned pins. The MCI in-

cludes 16 user deﬁned lines without pre-deﬁned use.

Only devices, which use these pins may connect to

them. The pins must be routed between both connec-

tors and the IC. But the devices must be placed as

adjacent devices, while a routing of UDEF pins is not

permitted on layers that do not use it.

SPI, as most common bus interface for low power

motes, is a synchronous serial data link operating in

full duplex mode. Devices will be differenced in a

master and a multiple number of slaves. Slaves are

addressed with individual select lines. The MCI de-

ﬁnes three data lines and one select line. Additional

slave could be addressed by a GPIO line. In an ac-

tive routing conﬁguration a slave must be connected

to the bottom side connector and a master to the top

side connector. By using a passive routing a module

can be a slave as well as a master.

C is another synchronous serial data link. In

contrast to Serial Peripheral Interface (SPI) I

C uses

only two lines, a data and a clock line, and is multi-

master capable. These properties signiﬁcantly sim-

plify routing, but require a more complex driver soft-

ware. Like all multi-master buses I

C requires exter-

nal pullup resistors for both lines. On a TandemStack

these resistors are placed on a bottom module or in

case of an active routing on the bus-breaking module.

TandemStack - A Flexible and Customizable Sensor Node Platform for Low Power Applications

Bottom Layer

Stack Layer

Top Layer

Figure 4: TandemStack JTAG chain. Devices are chained by connecting TDO with TDI between neighbored devices. The

chain must be closed by a top layer module. TMS and TCK are connected to all devices.

UART is a type of asynchronous re-

ceiver/transmitter. It requires at least two lines,

RX and TX, but is not a bus with a multiple number

of peers. An UART link is a strict point-to-point

connection with two peers. Due to this limitation

the TandemStack supports in a passive routing

conﬁguration only one UART between the modules.

For a multiple number of links an active routing must

be implemented.

Counter lines are only available on the connector

X2 and can be used as a counter input to accurately

measure event timing or frequencies.

4 APPLICATION SCENARIOS

Typical application ﬁelds for motes are very wide

and rapidly growing. They include sensing environ-

mental parameters, controlling or monitoring indus-

trial plants, buildings as well as public infrastructures.

Below we describe three current examples for our

TandemStack.

4.1 Low Power IEEE 802.15.4 Mesh

Network

In the IQlevel project a network of observation wells

for measuring the groundwater level in a drainage

area should be digitalized. As main project objec-

tive IQlevel mote must operated without a permanent

power supply and must be integrated in a 2-inch ob-

servation pipe. The distance among the nodes differs

from 100 up to 1.500meters and the territory changes

from free ﬁeld with a line of sight to forests with heav-

ily obstructed connections. Due to these requirements

and broad variation in the transmission range a ﬂexi-

ble mote design was needed. Figure 5 shows the re-

sulting round mote design with at least three layers

and an optional CC1190 ampliﬁer module. Besides

a battery, power supply can also be supplemented by

a alternative TEG module. To reduce material costs

ampliﬁers and TEG will only be deployed with a node

when its is needed.

Figure 5: IQlevel mote with battery power supply, MCU,

868MHz transceiver and ampliﬁer module.

4.2 Research and Development

Although the basic ideas of our TandemStack were

driven by the IQlevel project we realized during the

project time that our TandemStack is also very help-

ful for testing new ICs. At the IHP various research

groups are working on highly efﬁcient mote com-

ponents. Testing these devices requires a platform

where single components can be replaced easily. Our

TandemStack approach fulﬁlls all these requirements

better than the state of the art.

With our TandemStack topology it is possible to

test new MCUs or radio baseband processors by de-

signing a single PCB. Furthermore we can compare

our devices with commercially available ones by run-

ning similar software on the devices under test. The

adaptable TandemStack platform makes hardware de-

velopment processes more structured and efﬁcient

and therefore more cost effective.

4.3 WSN Bridge

Low power WSNs are the main use case of IEEE

802.15.4 based radio modules. But IEEE 802.15.4

low power and mesh network capabilities make a con-

nection to standard infrastructuresmore tricky and de-

vices are rare as well as expensive. In the context of

SENSORNETS 2012 - International Conference on Sensor Networks

the Trusted Sensor Node (TSN) project we design a

mote with an IEEE 802.15.4 and a bluetooth inter-

face. As illustrated in ﬁgure 6, by using a TSN device

a low power WSN can be connected to any bluetooth-

capable device, e.g. a simple handheld device. The

TSN uses an IPv4 protocol stack via its bluetooth in-

terface to external devices, so that integration in stan-

dard infrastructures is quite simple.

IEEE 802.15.4 sensor network

Bridge Mote

Figure 6: TSN mote as sink of a WSN, which is

equipped with two different transceivers and bridges an

IEEE 802.15.4 sensor network with a bluetooth handheld

device.

For running complex protocol stacks the TSN

mote should be equipped with a powerful 32-bit pro-

cessor. We chose a LEON-2 core, which was fabri-

cated in the IHP in house fabrication plant (fab). Be-

sides the LEON-2 core was extended with hardware

accelerators for cryptographic methods. For the blue-

tooth and IEEE 802.15.4 interface standard devices

were used. If motes and TSN are implemented as

TandemStack motes almost all layers can be reused.

5 MOTE’S COMPARISON

One important factor for the comparison of motes is

their respective processing power. Especially in large

scale nets with star topologies, the processing power

for data sinks and bridge nodes can become a bottle-

neck for network performance, whereas in small scale

networks this might be a waste of resources. Most

mote designs have a ﬁxed CPU that dictates its pro-

cessing power. Consequently, the network’s ability

and structure are chosen to ﬁt the node’s abilities, not

vice versa. IHP TandemStack solves this problem as

single node’s processing power can be suited to the

network structure and can be designed to be optimal

for the given challenge. For example if a star network

is desired, the center node can be as powerful as the

throughput demands and the single motes can imple-

ment a low-performance CPU.

Besides processing power, electrical and physical

characteristics are important values for mote’s usabil-

ity. We compared powerconsumption and footprint of

our TandemStack mote to ﬁve commercially available

motes. The results are summarized in table 3. The

TandemStack was built up with an MSP430-based

MCU module, an 868MHz radio module and was di-

rectly powered by 3.3V without DC/DC converter.

We can see that the smallest footprints are achieved

by stacked designs. The additional mote’s height is an

acceptable factor as most applications have a minimal

height, which allows packaging of two or three layers.

Furthermore, in comparison with a TmoteSky, we see

that a stacked design with its additional connectors

has a few µWatts higher idle consumption only.

Table 3: Comparison of mote’s footprint and power con-

sumption.

Mote Idle Footprint

TandemStack 29.7 µW 40x40 mm

IHPNode 8.4 mW 50x70 mm

TmoteSky 15.3 µW 32x66 mm

Mica2 75 µW 58x32 mm

Tyndall25 60 mW 25x25 mm

JN5148 12.4 µW 30x45 mm

Another crucial point for motes abilities is their

RF interface. Whereas most of them implement a

single radio or a number of radios to choose from,

our TandemStack can implement virtually any radio

on the market. This holds true not only in single

path communications, but also in multi-path designs.

Furthermore, our open interface opens the opportu-

nity to implement nodes that are not limited to ISM-

band communication only. Very specialized applica-

tions such as 403MHz medical implants, astronomi-

cal bands or even non-public bands and bridge appli-

cations between protocols and standards can be im-

plemented.

The IHP approach therefore also solves the prob-

lem of local radio permission authorities. Most motes

are limited to worldwide ISM-bands to give them the

minimum common intersection set between standards

worldwide. This leads to an increasing load of these

few international frequencies, which makes it harder

to set up a local network with standard equipment.

On the other hand there are readily available radios

in most countries that ﬁt in local standards and serve

bands that are usually not as heavily packed as inter-

national frequencies. The ability of the TandemStack

to meet international standards as well as to inherently

serve special demands of any given application makes

it one of the most versatile mote solutions so far.

Compared to other standard sensor nodes, the

TandemStack can be advantageous in space-limited

applications while still maintaining the highest level

of ﬂexibility. Whereas the design of other motes is

restricted, the modular IHP approach allows designer

TandemStack - A Flexible and Customizable Sensor Node Platform for Low Power Applications

to start developing application on a comfortably sized

development environment. At same time, as soon

as application is speciﬁed, it is possible to develop

a shrunk application platform, which uses previously

designed and tested software. This work ﬂow ensures

highest level of ﬂexibility and saves a lot of precious

development time.

6 FUTURE WORK

The interoperability of modules is tightly coupled

with a careful assignment of GPIOs. For example a

multiple use of GPIO lines must be avoided as any

SPI slave needs a dedicated chip select or interrupt

line. Our current set of TandemStack modules is still

surveyable. To keep an overview becomes more and

more difﬁcult with future extensions. A tool support

will signiﬁcantly simplify such a planning process.

Such a tool can help to ﬁnd a suitable combination of

modules or selection of lines for upcoming modules.

Driven by active projects a broad set of stack mod-

ules is already implemented. Nonetheless, some mod-

ules, which are very useful in research and typically

WSN scenarios, are still missing. Next steps will in-

clude the design of a storage or an FPGA module.

In a lot of application scenarios a low power stor-

age module with a capacity in the megabytes range

will be needed. Flash memories with SPI or I

C are

available and ﬁt perfectly to our TandemStack. Fur-

thermore, an FPGA as a high performance, reconﬁg-

urable processing core is therefore very useful for de-

velopment or required for massive data processing,

e.g. video stream encoding. It can be implemented as

an MCU replacement as well as a high performance

co-processing unit.

7 CONCLUSIONS

This paper described the TandemStack platform and

its Mote Component Interconnect (MCI). The

TandemStack follows the objective to assemble as

few components as possible needed for one functional

unit on a single Printed Circuit Board (PCB). We

assume that modules should only include one mote

component such as MCU, transceiver, power supply,

storage or sensor. Later the modules are connected

by a uniform MCI. We have explained how such

an interconnect can be deﬁned for building a ﬂexi-

ble mote platform where modules can be freely com-

bined. We compared our design with commercially

available motes and demonstrated that a stack archi-

tecture will provide the highest component density.

We illustrated that in contrast to the inﬂexible motes

our design can be used in various application scenar-

ios with minimal development effort. We have al-

ready implemented more than nine different modules

and could gather experimental results in real world

scenarios as well as in research activities. We are cer-

tain that a ﬂexible, standardized mote platform can

signiﬁcantly push development activities and mote

deployments.

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