A wireless sensor network (WSN) consists of spatially distributed autonomous sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants.
Recently, wireless sensor networks have found their way into a wide variety of applications and systems with vastly varying requirements and characteristics. A sensor node, also known as a ’mote’, is a node in a wireless sensor network that is capable of performing some processing, gathering sensory information and communicating with other connected nodes in the network.
Motes software often needs to be updated after deployment for a variety of reasons, such as fixing software bugs, modifying tasks of individual nodes or of the entire network, and patching security holes. Given the spatial distribution of the senor nodes among the field of deployment, the locations of the nodes may be unfixed or unreachable.
As a consequence, upgrading the sensor node firmware on every node in the field is a difficult job. Over-the-air programming (OTAP) protocols play a key role as an enabling technology to solve such challenges, and several protocols and algorithms have been specifically designed for this purpose.
OTAP in such motes supports the software update for the microcontroller resides on the processor/radio board inside the mote. However, in many applications the microcontroller jobs are split, such that a dedicated controller is used to interface with sensors while the main controller remains to handle the transceiver and advanced data processing jobs.
The idea described in this work is to enable firmware update of motes and sensor interfaces used in wireless sensor networks in their final locations from a central base station.
To that end, it makes use of SoC (Programmable System on Chip) devices which can be used as coprocessors for the motes' microcontroller. We describe the work on the development of PSoC based sensor interface board. Signal conditioning, filtering and amplification tasks are all preformed on the PSoC chip and can be modified and tuned by means of OTAP.
The configuration process of the PSoC (the 8051-based CY8C29466) is controlled via a separate CPLD chip. Our approach relies on the use of new generation of CPLDs (MAX II from Altera) which consumes very low power appropriate to the wireless sensor nodes power constraints.
This CPLD is used to perform the reconfiguration of the sensor interface controller; with this approach, the configuration data of this controller is transferred to the sensor node via the same RF link between the base station and the motes. These data is collected into flash memory and then used to program the target PSoC controller.
The PSoC in sensor interface boards gives higher performance and flexibility. Such motes can be used to build dynamically adaptable WSNs that can be modified to achieve a certain goal even after deployment. An example of reading a temperature sensor is illustrated; reconfiguring the PSoC over-the-air to adjust for a better accuracy over a different temperature range is also verified.
The main drawback of the currently used PSoC chip is the high power consumption. A new PSoC 3 family from Cypress is being considered. It includes a major power consumption reduction (330 µA at 1 MHz and 1 µA at sleep mode) and a low supply voltage (.5V – 5V).
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