Berkeley Motes

The Berkeley motes are a family of embedded sensor nodes sharing roughly the same architecture.

Let us take the MICA mote as an example. The MICA motes have a two-CPU design. The main microcontroller (MCU), an Atmel ATmega103L, takes care of regular processing. A separate and much less capable coprocessor is only active when the MCU is being reprogrammed. The ATmega103L MCU has integrated 512 KB flash memory and 4 KB of data memory. Given these small memory sizes, writing software for motes is challenging. Ideally, programmers should be relieved from optimizing code at assembly level to keep code footprint small. However, high-level support and software services are not free. Being able to mix and match only necessary software components to support a particular application is essential to achieving a small footprint. A detailed discussion of the software architecture for motes.

In addition to the memory inside the MCU, a MICA mote also has a separate 512 KB flash memory unit that can hold data. Since the connection between the MCU and this external memory is via a low-speed serial peripheral interface (SPI) protocol, the external memory is more suited for storing data for later batch processing than for storing programs. The RF communication on MICA motes uses the TR1000 chip set (from RF Monolithics, Inc.) operating at 916 MHz band. With hardware accelerators, it can achieve a maximum of 50 kbps raw data rate. MICA motes implement a 40 kbps transmission rate. The transmission power can be digitally adjusted by software through a potentiometer (Maxim DS1804). The maximum transmission range is about 300 feet in open space.

Like other types of motes in the family, MICA motes support a 51 pin I/O extension connector. Sensors, actuators, serial I/O boards, or parallel I/O boards can be connected via the connector. A sensor/actuator board can host a temperature sensor, a light sensor, an accelerometer, a magnetometer, a microphone, and a beeper. The serial I/O (UART) connection allows the mote to communicate with a PC in real time. The parallel connection is primarily for downloading programs to the mote.

It is interesting to look at the energy consumption of various components on a MICA mote. A radio transmission bears the maximum power consumption. However, each radio packet (e.g., 30 bytes) only takes 4 ms to send, while listening to incoming packets turns the radio receiver ON all the time. The energy that can send one packet only supports the radio receiver for about 27 ms. Another observation is that there are huge differences among the power consumption levels in the active mode, the idle mode, and the suspend mode of the MCU. It is thus worthwhile from an energy-saving point of view to suspend the MCU and the RF receiver as long as possible.

Source of Information : Elsevier Wireless Networking Complete 2010

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