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Ease wireless DAQ using integrated sensor nodes with GSM modems

Posted: 03 Apr 2012     Print Version  Bookmark and Share

Keywords:sensor network  data acquisition 

The operating systems for sensor networks resemble embedded operating systems since they are developed keeping an application in mind and are not generic. Also, since the system is built with low-power and low-cost capabilities, most general purpose operating systems must be eliminated. Given that most sensor networks do not require real-time capabilities, a smaller operating system such as TinyOS that has been specifically designed for sensor nodes may be used.

Figure 2 shows a typical implementation of a sensor network using a Global system for mobile communication (GSM) modem. Here all of the sensors communicate their data to a centralized server. The server has control over individual sensor nodes; however, individual sensor nodes cannot communicate between themselves. The server has to be involved for any communication between any two sensor nodes.

Figure 2: Typical implementation of a sensor network.

GSM modem
GSM is one of the standards for mobile telephony in the world. Although initially used only for voice communication, it has been adapted to include data capabilities by means of General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE). A GSM modem is a type of modem which accepts a SIM (Subscriber Identification Module) and operates like a mobile phone. GSM modems can be used in low power mode or can also be turned off when they are not in use.

The cost of transmitting data by GSM networks is has been falling rapidly. What's more, a GSM modem can easily be interfaced to microcontrollers using standard communication protocols. Mobile phones are increasingly handling data along with voice. Most GSM modems have a TCP/IP stack implemented on them and can be used to transmit data over secure channels. This also reduces the complexity of developing applications and enables the use of simple microcontrollers to interface to the GSM modem.

A GSM modem can connect to any IP (Internet Protocol) address and transmit data. Multiple modems can send data to a single IP address, and all the data can be collected and processed from a single location anywhere in the world. Users can dynamically configure each and every modem remotely based on data sent across from the network. In certain networks, a single node would have GSM capability. Other nodes would send their data to this particular node to be transmitted to the centralized server. This could reduce the cost of the overall system but would not be possible if nodes are not clustered together.

GSM modems also have the ability to provide instant alerts using SMS (short messaging service) or by transmitting data to a different, high-priority IP address based on certain conditions. These features can be used for fault tolerance and redundancy checks.

Let us consider an example of a GSM modem (SIM300) where communication happens over a serial port with the microcontroller. The modem has a standard set of commands called AT commands. These commands control the operation of the modem from the microcontroller. The microcontroller sends these commands over a UART (Universal Asynchronous Receive Transmit) interface at a specified baud rate.

The data sent through the serial port can be transmitted to a centralized server by configuring the modem using a specific set of commands. Thus, interfacing with the GSM modem simplifies data acquisition and processing in sensor networks.

Complete system implementation
A sensor node consists of an analogue signal chain and also requires a host of digital peripheral interfaces. Also each sensor node may be interfaced to different kinds of sensors, requiring flexibility in its interfaces and I/Os. Programmability of individual nodes plays a critical role in a successful implementation of a sensor network.

Mixed-signal programmable microcontrollers like PSoC from Cypress provide analogue and digital sub-systems which can be configured to provide the functionality required by individual nodes. This eliminates the need for specialised hardware for each different kind of sensor. Such System-on-Chip-based (SoC) MCU also have the capability to handle all tasks required for the sensor node in a single chip, including ADC, DAC, PGA's, comparators, op-amps, digital filtering capabilities, DMA, and LCD controllers, among others.

Design of the system using SoCs allows frequent and fast changes in design to assist customisation of sensors nodes. PSoC with its low-power capabilities at sleep mode is very suitable for these applications.

About the authors
Ajay Bharadwaj is a senior applications engineer at Cypress Semiconductor.

Balaji Mamidala is an applications engineer at Cypress Semiconductor.

To download the PDF version of this article, click here.


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