Ember is another long-time participant in the field. Its latest ZigBee systems-on-a-chip (SoCs), the EM351 and EM357, include a full 802.15.4 2.4-GHz radio with ZigBee protocol stack. They also include a 32-bit ARM Cortex M3 processor to run the application. The EM351 has 128 kbytes of flash, while the EM357 offers 192 kbytes.
With a power output in the +3- to +8-dBm range and a receiver sensitivity of –102 dBm, the link budget is exceptional. Power consumption is low. With good power management, battery life can last many years. Users can obtain a development kit radio module using the EM35x (Fig. 3). One of the most common applications for the Ember modules is in HANs (Fig. 4).
An interesting proprietary technology comes from EnOcean, whose Dolphin platform was designed for building automation, home networks, and other systems requiring very low power consumption and long life. The radio technology uses the 868-MHz band or the 315-MHz band. Even at low power, practical ranges are possible because of the low-frequency design.
Typical range within buildings is 30 m, but up to 300 m can be achieved over a free-space path. The data rate is 125 kbits/s, transmission may be one-way or two-way, and a unique 32-bit ID is used. The basic radio modules (Fig. 5) can operate without batteries using three types of energy harvesting:
• Mechanical: A magnet and coil inside a light switch generates power each time the switch is actuated. A self-powered light switch generates power and converts it to a radio signal every time the light switch is pressed.
• Solar: Most of the sensors (occupancy/motion, door/window, photo/light) are powered by collecting and storing energy from light. When combined with smart and ultra-low-power radios, sensors can operate with just 40 lux of ambient light. (In a typical indoor setting, more than 400 lux is usually available.) The energy is stored in capacitors, which allows the sensors to do their job even when they’re in complete darkness for days.
• Thermal: When energy is needed to control sensors residing in permanent darkness, temperature differentials can generate energy for wireless communications. This is the newest form of micro energy harvesting, and it’s enabling self-powered controls such as valve actuators.
The Z-Wave products from Sigma Designs (formerly Zensys) are also unique. Using a mesh architecture, the nodes can be used with switches, lights, thermostats, and appliance controllers. They are also compatible with some of the Advanced Metering Infrastructure (AMI) electric meters being installed as part of the Smart Grid initiative to manage and control energy usage in the home.
The Z-Wave modules operate on 908.42 MHz in the U.S. using FSK modulation and can deliver a data rate of 9.6 or 40 kbits/s as needed. The Z-Wave ZM3102N’s 8051 controller runs the protocol and mesh network. With low power, battery life can be very long. Dozens of companies use the Z-Wave modules for home monitoring and control, such as the Z-Wave-enabled Trane thermostat (Fig. 6). These and other end products are widely available in Lowe’s and Radio Shack stores.
Microchip Technology has a line of 802.15.4/ZigBee products as well as some low-power ISM-band radio chips. But the company’s recent acquisition of ZeroG Wireless included a WSAN product called Wi-Fi I/O. The primary product is the ZG2100, an 802.11b Wi-Fi module designed for very low power consumption.
The ZG2100 runs the standard .11b protocol but speed is limited to 1 or 2 Mbits/s. It is Wi-Fi certified and runs the available WEP, WPA, or WPA2 security. Also, it uses a serial peripheral interface (SPI) and is only 21 by 31 by 3.7 mm. If you need the speed as well as low power consumption, this is an attractive option. And, it’s very easy to incorporate into existing LANs.
One of the more interesting new products to address the WSAN market is Silicon Laboratories’ Si10xx wireless MCU family. This series of devices packages an ISM-band radio along with an 8051 controller, giving designers multiple ways to design their product. A unique power system with an efficient low-dropout (LDO) regulator and dc-dc converter adds a new dimension to the need for low power consumption and super-long battery life.
The top-of-the-line device is the Si1000, which features a 25-MIPS 8051 with 64 kbytes of flash and the usual mix of I/Os and interfaces as well as timers. A 10- or 12-bit ADC is also on chip as well as a temperature sensor and voltage comparators.
The radio is a real gem. It can be programmed to operate over the 240- to 960-MHz range, which covers the standard ISM frequencies of 315, 433, or 868 MHz. Modulation is FSK or GFSK with a data rate to 250 kbits/s. The receive sensitivity is an amazing –121 dBm while the programmable transmit power can be up to 20 dBm for a net link budget of 141 dB. This can extend range up to 3 km over a clear line-of-sight path.
The big news is the internal LDO and dc-dc converter with their programmable power-management unit, which keeps total power consumption low under all possible operating conditions. The EZMac software lets designers create a protocol for point-to-point, multipoint-to-point, and simple mesh networks. The Wireless M-Bus software, also available, is widely used for metering in Europe. Availability is scheduled for the second quarter of this year.
DESIGN ISSUES
The main design issues for WSANs vary depending on the applications, but three stand out: power consumption, ease of network modifications, and security.
Because the nodes are battery operated, long life is essential to minimize the time, cost, and inconvenience of changing batteries. Some of the newer modules offer a battery life of years, though most are considerably less. Look for products that transmit data in packets at high speed to minimize transmitter on time. A short transmit duty cycle is essential to long battery life.
Next, how easy is it to remove modules or add modules? The most desirable situation is one in which the system is ad hoc and modules may come and go without any reprogramming or intermediation.
Finally, security may be an issue in your application. Most standards provide some level of security, but you have to verify that it is sufficient for your application.