A 3D miniaturised programmable transceiver

A 3D miniaturised programmable transceiver Brendan O’Flynn, S. Bellis, K. Mahmood, M. Morris, G. Duffy, K. Delaney and C. O’Mathuna Tyndall National Institute, Cork, Ireland Abstract Purpose – To describe the development of a three dimensional programmable transceiver system of modular design for use as a development tool for a variety of wireless sensor node applications. Design/methodology/approach – As a stepping-stone towards the development of wireless nodes, sensor networks programme was put in place to develop a 25 mm cube module, which was modular in construction, programmable and miniaturised in form factor. This was to facilitate the development of wireless sensor networks for a variety of different applications. The nodes are used as a platform for sensing and actuating through various parameters, for use in scalable, reconfigurable distributed autonomous sensing networks in a number of research projects currently underway in the Tyndall Institute, as well as other institutes and in a variety of research programs in the area of wireless sensor networks. Findings – The modular construction enables the heterogeneous implementation of a variety of technologies required in the arena of wireless sensor networks: Intelligence, numerical processing, memory, sensors, power supply and conditioning, all in a similar form factor. This enables rapid deployment of different sensor network nodes in an application specific fashion. Research limitations/implications – Characterisation of the transceiver module is ongoing, particularly in the field of the wireless communication platform utilized, and its capabilities. Practical implications – A rapid prototyping and development cycle of application specific wireless sensor networks has been enabled by the development of this modular system. Originality/value – This paper provides information about the development work and some potential application areas made available by the implementation of a miniaturised modular wireless sensor node for use in a variety of application scenarios. Keywords Sensors, Transceivers, Electric power transmission Paper type Research paper

This module can be used in such applications areas as tangible autonomous agents, spiking neural network controllers, digital signal processing, wireless data communication, wireless sensor networks, home automation, automotive and industrial sensors depending on the application specific configuration implemented.

1. Introduction There is a drive towards wireless nodes for sensor networks of volume in the sub 5 mm region as part of the NMRC Ambient Electronic Systems group technology development roadmap. As a stepping-stone towards this goal, the target objectives for the development of the 25 mm cube module were to develop a low volume prototyping and experimentation platform. This is for use, as a platform for sensing and actuating through various parameters, for use in scalable, reconfigurable distributed autonomous sensing networks in a number of research projects currently underway in the NMRC, as well as other institutes in a variety of research programs in the area of wireless sensor networks (Plate 1). This paper outlines the development and characterisation of the novel configurable modular miniaturised system, giving details on current research project application areas. The development of the various system layers is described, and the difficulties involved in the development of a highly miniaturised modular system outlined. These innovative modules have the capability to interact, respond and learn from their surroundings making integration of engineering, computer science and human intelligence a reality.

2. Design methodology 2.1 Stackable/modular layers Project requirements in the NMRC, in the areas of autonomous, mobile sensor networks for ambient electronics have necessitated the development of different modules to integrate system electronics in various configurations. To this end a modular system was developed to enable “plug and play” interoperability between different functional blocks for a highly miniaturised (25 mm) system. The modules use a stackable connector system to make the electrical and mechanical interconnections between layers. These high-density connectors have 0.5 mm pitch and are available in range of interlayer spacing from 5 to 8 mm to allow different component heights on the PCBs. The connectors facilitate an 80 pin general purpose bus and a 40 pin bus for configuration and data transfer between the module layers. Layers developed include a microcontroller/RF transceiver layer, a FPGA layer, a sensor interface/ communications layer, a thermistor based sensor layer, an I2C based temperature sensor layer and a power supply layer. The stackable configuration enables ease of connectivity between the layers depending on the system level requirements (Figure 1).

The Emerald Research Register for this journal is available at www.emeraldinsight.com/researchregister The current issue and full text archive of this journal is available at www.emeraldinsight.com/1356-5362.htm

Microelectronics International 22/2 (2005) 8–12 q Emerald Group Publishing Limited [ISSN 1356-5362] [DOI 10.1108/13565360510592162]

This paper was first presented at the XXVIII International Conference of IMAPS Poland Chapter, Wroclaw, 26-29 September 2004.

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A 3D miniaturised programmable transceiver

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Volume 22 · Number 2 · 2005 · 8 –12

Plate 1 Final configuration of 25 mm system showing four different stacked functional layers

2.3 RF transceiver layer parameters To provide wireless communications capability between sensor nodes, a transceiver/microcontroller layer was developed. This incorporates a microcontroller and transceiver transmitting in the 2.4 GHz ISM band. This layer can be used as a “stand alone” system layer, using the processing power in the microcontroller for system control, communications protocols and limited number crunching capability. This programmable transceiver has been designed to connect with the separate battery module and FPGA and sensor layers depending on the configuration required by the end user or his mobility/portability requirements. Figure 3 shows the top and bottom views of the transceiver PCB along with a block diagram showing the interconnection of the main components on the module. The transceiver (the nRF2401 from Nordic VLSI) consists of a fully integrated frequency synthesizer, a power amplifier, a crystal oscillator and a modulator. Output power and frequency channels are easily programmable. Current consumption is very low, and a built-in power down mode makes power saving easily realizable. The module also features an on board 50 V antenna. The embedded microcontroller is based on the ATmega128L[2], an 8-bit microcontroller with 128 KB in-system programmable flash. The user can easily program the device with custom protocols for use in his end product or for general product development. TinyOS[3] is an operating system designed at UC Berkeley engineered to run in hardware platforms with severe resource constraints, and is directly importable into the program memory of this device, thus enabling the development of complex protocols for use in power constrained sensor networks. As the system is envisaged to operate in mobile sensor applications, low power consumption of the system is essential. Power consumption considerations were taken into account from the beginning of the design phase of the system. The transceiver selected (nRF2401[4] from Nordic VLSI) is able to operate in “Shockburste” mode. This uses on-chip FIFO to clock in data at a low data rate and transmit it at a very high rate thus greatly reducing power consumption. Putting all high speed signal processing related to RF protocol into the nRF2401 reduces the current consumption, lowers system cost (by facilitating the use of a less expensive microcontroller), and greatly reduces the risk of “on-air” collisions due to short (high speed) transmission time. The Atmel microcontroller can be programmed to operate in sleep or powerdown mode awaiting activity on an interrupt pin (i.e. data has arrived, or some alarm condition reached).

Figure 1 Block diagram showing project concept and stacked layers

The RF transceiver layer also has a separate 20 pin connector for four low noise analog input channels so as to have the capability for integrating analog sensors directly to the microcontroller part of this section. For developmental purposes a number of different layers have been constructed for use in various projects. Due to the modular nature of the system, and the plug and play capability of the stacking system developed, this 25 mm platform has proved to be an invaluable developmental tool in the evaluation of power constrained networking protocols, ambient system networks, transducer networks and mobile sensor based systems, particularly those requiring wireless communications. 2.2 FPGA layer parameters In order to have a high-speed DSP type processing capability in the system, a FPGA based layer was developed in the 25 mm footprint. The FPGA PCB contains a Xilinx Spartan IIE Field Programmable Gate Array (FPGA)[1]. The Spartan IIE 1.8 V FPGA family gives high performance, abundant logic resources, and a rich feature set. The device integrated into the 25 mm PCB series is the XC2S300E-7FG256. This is a mid-range device with density of up to 300,000 system gates. Features include dedicated block RAM, distributed RAM, programmable I/O and delay-locked loops (DLLs) for minimization of clock skew. The FPGA avoids the initial cost, lengthy development cycles, and inherent risk of conventional ASICs. Also, FPGA programmability permits design upgrades in the field with no hardware replacement. The module features an on board 4 MHz crystal oscillator chosen to give a moderate processing rate while conserving power. A 1.8 V and 3.3 V low drop out power supply regulators provide the maximum module lifetime from a coin cell battery attachment for the core and LVTTL I/O voltage requirements, respectively. The module also features an onboard Flash serial EPROM such that the FPGA configuration memory is automatically downloaded on power-up. The stackable connector system used allows simple connectivity to other modules such as the RF module, coin-cell power supply and sensor modules (Figure 2).

2.4 Integration and signal layer conditioning parameters To enable integration of various sensor types and communication to the system, a communication and sensor interface to the FPGA module, or the transceiver module was developed. It contains a dual channel RS232 transceiver, the maxim MAX3224ECAB[5] in a small SSOP20 package enabling wired serial communication with a PC or a PDA for example. The module also contains two TLC549CD analogue to digital (A to D) converters from Texas Instruments[6] for interfacing analogue sensors to the FPGA. The module also allows interfacing to seven external sensors, which change their resistance according to the 9

A 3D miniaturised programmable transceiver

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Volume 22 · Number 2 · 2005 · 8 –12

Figure 2 Block diagram and final implementation of RF transceiver/microcontroller layer

Figure 3 Block diagram and final implementation of RF transceiver layer

parameter being measured (e.g. Thermistors or light dependent resistors (LDRs)). Digital based sensors can be hooked up directly to the IO connector, the transceiver module, or the FPGA.

Plate 2 Solar cells wrapped around 25 mm module and sending signal to activator

2.5 Battery layer parameters Coin cells are used to provide power to the 25 mm system when used as a wireless mobile node in a sensor network. A power layer has been designed to directly interface coin cells using the stackable connector system. A range of options exist for choice of coin cell including support for 20 and 24.5 mm cells. Options are available with capacities from 130 mAh for configurations that draw little power, to capacities of 560 mAh for the more demanding applications. and connected to various rechargeable batteries so as to evaluate their performance under different conditions. The performance of a number of solar cells in diverse light intensities were investigated and a number of tests were performed on these solar cells. In order to increase their voltage output and current output, the cells were connected in series and in parallel and their outputs were examined. The batteries that were chosen to examine their capabilities when connected to a solar cell were three Nickel Metal Hydride (NiMH) batteries with capacities 20, 450 and 600 mAh and two coin cell Lithium Manganese dioxide rechargeable batteries with capacities 25 and 65 mAh. These batteries were chosen, as they were commercially available and also environmental friendly. The coin batteries are best suited towards the 25 mm cube module as they can fit perfectly into the battery holder on the top layer of the 25 mm cube. The other NiMH batteries are examined so as to compare their performance with the coin cell batteries and highlight their performance for future references.

3. Energy harvesting for wireless sensor networks – solar cell analysis In order to power the 25 mm cube; solar power as an energy conversion device and rechargeable batteries as an energy storage device were investigated. The combination of a solar cell and a rechargeable battery is known as a “hybrid power supply”. The use of the solar cell is to recharge the rechargeable battery, which obviates the need for an external power connection and makes autonomous operation possible. The rechargeable battery is used to provide power to the application. So in this hybrid system the solar cell is used to meet standby needs and to recharge the battery as needed and the battery is used to supply peak power by storing the energy from the solar cell. The solar cells chosen for investigation were the IOWA 115 mm £ 24 mm solar cell and the Solems 17 mm £ 13 mm solar cells. These were placed around the 25 mm cube micromodule (Plate 2) 10

A 3D miniaturised programmable transceiver

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Volume 22 · Number 2 · 2005 · 8 –12

The batteries connected in parallel with the different solar cells were placed in a light irradiance of 30,000 Lux (1.98 W/ mm2), which is equivalent to an overcast day and a light irradiance of 450 Lux (27 mW/mm2), which is equivalent to office light. Their performance was investigated.

The solar cells in a light intensity of 30,000 Lux shows good performance and in brighter light such as 60,000 Lux (396 W/m2) would perform even better. As can be seen for autonomous wireless sensor nodes (Figure 4), the best solar cell/battery combination as regards the 25 mm module is two IOWA powerfilm solar cells connected in series with a 3 V 25 mAh battery. These solar cells connected in this manner are able to supply nearly 105.14 mJ of energy, which means theoretically it would keep a 3 V battery charged if the application used consumed less than 105 mJ of energy in a typical eight hour day.

3.1 Solar cell analysis results The rechargeable batteries require an energy source with a voltage greater than its nominal voltage for the charging to take place. When a battery is connected to a particular solar cell, the impedance of the battery affects the impedance of the solar cell so that the voltage produced by the solar cell is greater than the nominal voltage of the particular battery used. So when a solar cell is connected to a 3 V battery the solar cells impedance changes so that the solar cell produces a voltage output of over 3 V, if it is capable of doing so in the particular light intensity. That is why the solar cell modules were connected in series so as to gain this voltage output. It can be seen, that indoors these solar cells are unable to recharge 3 V batteries, as they have a low voltage output in this light intensity even when connected in series. When the solar cells are connected in parallel outdoors these solar cells are unable to recharge the battery as their operating voltage is not quiet high enough, whereas when connected in series it is high enough. In a light intensity of 450 Lux (2.97 mW/m2), the IOWA – ThinFilm PowerFilm Solar Cells as well as the SOLEMS solar cells are unable to recharge the 3V coin cell batteries making them unsuitable to be used with the 25 mm module. It can be seen that each of the setups of the solar cells are able to recharge the NiMH batteries. The solar cells in this light intensity of 450 Lux (2.97 mW/m2) connected to the different batteries shows poor performance. In a light intensity of 30,000 Lux (198 mW/m2) the IOWA ThinFilm PowerFilm Solar Cells are able to recharge the 3 V coin cell batteries when they are connected in series making them suitable to be used with the 25 mm module. In this light intensity the energy output performance increases, for example the IOWA connected in series setup is able to supply up to 105.14 J of energy. It can be seen that the solems solar cells when connected in series are able to recharge the 3 V coin cell battery also but the amount of energy it can supply is considerably lower about 24.7 J than the IOWA connected in series setup. This is because it is supplying less current in this light intensity to the rechargeable battery. It can be seen that each of the setups of the different solar cells are able to recharge NiMH 1.2 V batteries, the amount of energy it can supply in 8 h can be seen in the graph in Figure 4, highlighting what applications they are suited for.

3.1.1 Calculations It is considered that the module was placed in a bright sunlight of 60,000 Lux and it was activated about eight times in 24 h. The power (Ps) delivered by the battery is calculated to be 6 mW, when in sleep mode from equation (1). If the thermistor is activated eight times for 5 s each time in a 24 h day, then the power (PAct) consumed from the battery is 24 mW and the energy consumed each time is 120 mJ. It is activated eight times once every 3 h so it is activated for about 40 s of the 24 h. So the total energy (Es) consumed by the module when in sleep mode is calculated using equation (2) (518.16 J) where TDay is the total amount of seconds in 24 h and TAct is the total amount of seconds the module is activated. The total amount of energy consumed from the battery when the thermistor is activated is calculated using equation (4). The total energy (ETotal) consumed by the module is calculated using equation (5), which is found to be 519.12 J. In a light intensity of 60,00 Lux, the solar cell setup is capable of supplying 2 mA of current at a voltage of 3.5 V. In order to keep the rechargeable battery charged the solar cell needs to be placed in a light intensity of 60,000 Lux for 21 h and only be in darkness for 3 h. The total energy that the solar cell would be able to supply would be 529.2 J. This indicates that the solar cell would be able to provide enough energy to the rechargeable battery in order for the 25 mm module to operate indefinitely depending on the lifetime of the rechargeable battery and solar cell.

Figure 4 Solar cell and rechargeable battery indoor (450 Lux) and outdoor results

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P s ¼ Voltage £ Current

ð1Þ

E s ¼ P s ðT Day 2 T Act Þ

ð2Þ

P ACT ¼ V £ I

ð3Þ

E Act ¼ P Act £ T Act

ð4Þ

E Total ¼ E s þ E Act

ð5Þ

A 3D miniaturised programmable transceiver

Microelectronics International

Brendan O’Flynn et al.

Volume 22 · Number 2 · 2005 · 8 –12

Figure 5 Original e-chair system electronics and miniaturised version

4. Example application areas

required, or in a truly automated environment as the final scenario is envisaged to be; the receiver basestation would automatically commence irrigation until the temperature alarm ceases.

4.1 FPGA application – E-Chair As part of the eGadgets[7] project funded by the European Community under the “Information Society Technologies” programme, intelligence is to be integrated into every day objects, to interact with users unobtrusively and develop ambient systems. To this end, FPGA intelligence was integrated into a chair – the E-Chair (Kameas et al., 2003) as shown in Figure 5. The electronics is mounted on a perspex flap, which folds down from underneath the chair seat for maintenance. The sensors used are nine LDRs embedded in an array on the surface of the chair. When one or more of the sensors are covered, then its resistance changes, indicating that somebody is sitting on the chair. Signal conditioning is required before sending the data to the FPGA board, which detects a change in the sensory input and determines whether or not a signal indicating the chair status needs to be sent through the RS232 PCB to a PDA. As seen in Figure 5, this was initially developed as a wired system using standard printed circuit board and surface mount technologies. The 25 mm FPGA/RF/Comms layers provide an ideal opportunity for miniaturisation of the system, and the integration of the system electronics actually into the chair itself. Wireless communication is then possible between the e-Chair and other nodes on the network (PC, other eGadgets, etc.) via the GASOSe operating system (Kameas and Ringas, n.d.) developed as part of the eGadgets project.

5. Conclusions/future work A modular programmable transceiver based system for wireless sensor applications has been developed to fit in a 25 mm footprint. Based on the atmel family of devices the “TinyOS” operating system can be ported into the devices for development of different media access control (MAC) layers for use in various networking applications. This modular system has enabled the RF transceiver/FPGA/sensor system to be integrated into different applications depending on requirements. An analysis of potential energy harvesting techniques was undertaken for autonomous wireless sensor network applications. Additional work is underway to implement a RF transceiver layer in different ISM bands (433 MHz and 868/915 MHz), as well as, to implement a zigbee standard transceiver layer in the 25 mm form factor.

Notes 1 2 3 4 5 6 7

4.2 “E”graculture sensor application A thermistor layer for agricultural use was developed. This is a low power consumption temperature sensing circuit for potential use in a vineyard application to monitor leaf temperature (an indicator of the need for irrigation). The circuit was designed to operate using minimal current (micro amps) and to activate an alarm on over-temperature. This alarm was connected to the RF transceiver layer on a interrupt pin, so as to enable the system to operate in low power consumption “sleep mode” until an over-temperature alarm was sensed. The transceiver then transmits the alarm to a base station to alert the user that irrigation of the vine is

www.xilinx.com www.atmel.com http://webs.cs.berkeley.edu/tos/ www.nvlsi.no www.maxim-ic.com www.ti.com cswww.essex.ac.uk/research/iieg/egadgets.htm

References Kameas, A., Bellis, S., Mavrommati, I., Delaney, K., PoundsCornish, A. and Colley, M. (2003), Architecture that Treats Everyday Objects as Communicating Tangible Components, PerCom2003, Dallas-Fort Worth, TX. Kameas, A. and Ringas, D. (n.d.), GAS: An Architectural Style for Ubiquitous Computing that Treats Everyday Objects as Communicating Tangible Components, Computer Technology Institute.

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