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Abstract
In order to create a low-power and lightweight wireless sensor node for the control of MEMS microrobots, the Single Chip Mote project aspires to integrate a fully-functioning microprocessor, radio, sensors, and solar cells onto a single die, while also eliminating the need for external components through careful architectural design. This report presents the past two years of work on the design of the Single Chip Mote digital system, complete with an ARM Cortex-M0 microprocessor, control logic for an IEEE 802.15.4 radio, special-purpose radio timers, and ADC interface. This includes details on the design and contents of the Verilog code used to describe the hardware, and the software written to run and test the Single Chip Mote digital system. The required tools and testing procedures are also explained, along with the details required to convert this FPGA-based design to an ASIC design ready for tapeout. The intention behind this report is to pass on the knowledge acquired throughout the course of this project to those who are working to improve and
iterate on this design. This report also presents preliminary power, area, and timing characteristics for the ASIC version Single Chip Mote digital system.
Chapter 1: Introduction
The term “Smart Dust” was originally coined by Professor Kris Pister to describe low-cost, low-maintenance, and unobtrusive wireless sensor nodes on a micro scale.
These motes form interconnected mesh networks to communicate with one another and transmit sensor information.
Wireless Sensor Nodes and the Internet of Things
The recent rise in the popularity of the Internet of Things (IoT) has fueled the demand for consumer-quality wireless sensor node devices.
The wireless standards used in laptops and cell phones such as WiFi and LTE are too power-hungry to be used on small wireless sensor nodes.
Bluetooth Low Energy is appealing due to its compatibility with laptops and cell phones, at the cost of the
associated licensing fees. It also does not support the creation of mesh networks.IEEE Standard 802.15.4, entitled Low-Rate Wireless Personal Area Networks (LoWPAN), is also a popular choice since it defines the PHY and MAC layers underlying many other protocols commonly found on commercial motes. This standard is designed specifically for short-range, low-power, and low-data-rate application, and does not require licensing.
The OpenWSN project aims to create an open-source implementation of the complete protocol stack for IoT wireless sensor nodes with 802.15.4 radios. OpenWSN is compatible with a variety of software and hardware platforms, allowing different motes to communicate with one another and form mesh networks.
When Small is Not Small Enough (previous work)
Many of the commercially-available general-purpose motes are certainly capable of running OpenWSN and other applications within a mesh network. While these motes are small, the weight of the battery and PCB itself makes them too large and heavy to be used for microrobotic control and communication. Without the benefit of energy harvesting, these motes also need to have their batteries replaced every few days or perhaps weeks. Examples of these motes include the TelosB and OpenMote-CC2538, both with OpenWSN support [14].
Research projects involving low-power motes tend to focus more on cramming commercial hardware onto tiny PCBs than in new embedded architectures for low-power applications. The designs presented in [27], [28], and [7] are coin-sized, low-power, and unobtrusive motes optimized for infant observation, energy sensing, and transportation monitoring. Given the simplicity of their microprocessors, these motes are not able to implement a complex protocol stack for mesh networks. [27] and [7] also require their own base stations to communicate with the motes, whereas motes using IEEE 802.15.4 radios can communicate with any other mote or base station with an 802.15.4 transceiver. While these designs
succeed in lowering energy consumption, they still require batteries that may only last for a few weeks, and the combination of the PCB and battery is still too heavy for a microrobot.The authors of [18] claim to have developed the world's smallest wireless sensor node by designing a custom IC for their signal processing and data transmission. The custom IC die is directly bonded to a MEMS die containing all of the required sensors. The mote uses solar cells in combination with a rechargeable battery for longer battery life. This mote is small, lightweight, and has minimal external components. However, the major downside of the design in [18] is the lack of a general-purpose microprocessor, as it is designed for the sole purpose of sampling and transmitting data.
Perhaps the best attempt thus far towards full integration is the Michigan Micro 10Mote [20]. With as many as eight different layers containing the microprocessor, radio, sensors, and other components, the mote measures at just 2×4×4mm , and has an incredibly low standby current of 2nA. The mote can be powered completely via ambient light through its solar cells, and contains a battery layer to store any excess harvested energy. The only potential downside to this design is the increased complexity when manufacturing, aligning, and bonding eight different dies.
Finally, the 24/60GHz passive radio designed at Berkeley [31] proves that a low-power radio relying entirely on energy harvesting is possible on a single die. Unfortunately, the chip behaves more like an
RFID tag instead of an autonomous computer. As a result, two of these radios cannot directly communicate with one another, and are not well-suited for forming a network of microrobots. Also, both the 24GHz receiver and 60GHz transmitter are not compliant with any current IoT standards.
Single Chip Mote to the Rescue
The high-level block diagram in the figure below shows the various subsystems that must be integrated onto a single die in order for this project to succeed.
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The Single Chip Mote is the ideal microcontroller for the future swarms of autonomous microrobots, each with a lightweight yet fully-capable brain for performing actions beyond the simple observe and report.
The addition of the OpenWSN protocol stack allows for these robots to create an extensive and adaptive
mesh network, and communicate with a variety of IoT hardware platforms and sensors supporting OpenWSN.
Single Chip Mote Digital System
A tested and functioning FPGA prototype of the Single Chip Mote digital system complete with an ARM Cortex-M0 microprocessor, radio controller, custom radio timer, and ADC interface is presented, along with the tools and procedures for designing hardware, writing software, and verifying functionality.
A high-level block diagram of the Single Chip Mote digital system is shown below. This is far from the final iteration of the Single Chip Mote digital system; the design lacks support for integrated sensors, periodic sensing without intervention from the microprocessor, power management and low-power modes, and other potential hardware accelerators to handle repetitive and energy-consuming tasks normally executed in software.
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As an example of hardware acceleration, OpenWSN uses hardware timers to wake up the microprocessor for each step involved in sending a packet over the radio. The current Single Chip Mote digital system has custom timers designed to automatically trigger the actions required to send a packet without waking up the microprocessor, and the overall process requires less energy than a traditional microcontroller.
Preliminary Results
Preliminary results already show the potential for improvement when using the Single Chip Mote in place of existing wireless sensor nodes. The technology used for the complete design is TSMC 65nm LP, with a clock frequency of 5M Hz and an operating voltage of 1.2V.
The main reason for the improvement is most likely due the use of the 65nm LP processes.
The Single Chip Mote digital system also has significantly fewer on-chip peripherals than the MSP430 or CC2538, reducing both dynamic and leakage power.
Area is also an important consideration, since this chip must be light enough to be carried by a MEMS microrobot. The preliminary design for the Single Chip Mote digital system has a total cell area of 856600µm2, which easily fits within a die area of 1mm2 . Assuming an incident power of 1mW per mm2 in direct sunlight, CMOS solar cells with at least 10% conversion eficiency should be able to provide 100µW per mm2 of die area in direct sunlight. Therefore, this design (when run with an operating voltage 0.9V ) requires approximately 1mm2 of solar cells to power the Single Chip Mote digital system. It is estimated that the analog, radio, and voltage converters for the Single Chip Mote will require 2mm2 of area for the circuits themselves, and 2mm2 of area for additional solar cells. With these numbers in mind, the Single Chip Mote requires a total die area of 6mm2.
Given that the thickness of the die is about 200µm, and the density is similar to that of crystalline silicon (2.33g/cm3), the estimated mass of the die is 2.8mg .
Researchers in our group are currently designing MEMS motors and legs for walking microrobots. Each leg outputs a downward force of 300µN , and can move a mass of 30mg . The mass of the legs themselves are 15mg each, which allows for 15mg of payload per leg. With these values in mind, a one-legged MEMS microrobot generates enough downward force to support the weight of the Single Chip Mote.
Report Outline
Chapter 2 provides an overview of the tools for hardware development for the FPGA and software
development for the Cortex-M0 microprocessor, including the basics of their installation and use.Chapter 3 contains a detailed explanation of the Single Chip Mote digital system hardware.
Chapter 4 demonstrates how to write software that uses the hardware peripherals.
Chapter 5 covers the details on loading software onto an FPGA or ASIC containing the Single Chip Mote digital system.
Chapter 6 describes the current testing procedures, including simulation and real-time verification.
Chapter 7 details the changes required to convert the Single Chip Mote digital system from an FPGA design to an ASIC design.
Chapter 8 concludes this report with a discussion the accomplishments of this project and areas for improvement.
Chapter 2: Getting Started
Important terms
MEMS microrobot: micro-electro mechanical system microrobot
Review
Protocol Stack
The standards under development most applicable for the Internet of Things are:
The IEEE802.15.4e standard defines MAC amendment to the existing IEEE802.15.4-2011 standard. One mode, called Time Synchronized Channel Hopping, significantly increases robustness against external interference and persistent multi-path fading, while running on legacy IEEE802.15.4 hardware.
The IETF 6TiSCH working groups standardizes mechanisms of running an IPv6-enabled protocol stack on top of IEEE802.15.4e TSCH.
The IETF 6LoWPAN working group has standardized a mechanism for an IPv6 packet to travel over networks of devices communicating using IEEE802.15.4 radios; this includes header compaction techniques to fit long IPv6 headers into short IEEE802.15.4 frames.
The IETF ROLL working group has standardized the RPL routing protocol, i.e. the distributed algorithm which finds the multi-hop path connecting the nodes in the network with a small number of destination nodes.
These standards can be layered one on top of another, forming the following protocol stack:
application | |
transport | |
IP/routing | |
adaptation | |
medium access | |
phy |