Narrow band equipment has started to draw a lot of attention, since despite a low bit rate, it is resistant to ambient noise and can be used to communicate over long distances even where there are obstacles. There is also an increasing demand for data transmission from sensors and for actuator control that does not require the higher data capacity offered by wideband solutions. New applications are discovered. For example, mad-cow disease affected cattle could be monitored, as some experiments show, by attaching a body temperature sensor to each cow and monitor them centrally. If body temperature or electro-cardiogram changes are discovered, an immediate response is possible so that the situation can be prevented from getting out of control. In the home and in offices, and in public facilities such as hospitals, train stations and the like, the number of areas in which information is required from a variety of sensors is growing. However, usage inside facilities in urban areas is always beset with problems such as radio wave fading and ambient noise. In addition, these radio units are required to have low consumption current, low voltage, and to be small, light and durable with flexible working temperature, demands which create headaches for system designers.
Design challenges
In most telemetry applications, high quantities of information are not needed, although in many cases robust transmission and product reliability are required. The capabilities of narrow band equipment coincide closely with these requirements.System developers tend to view radio modules as simple components. But when designers actually develop a system using conventional radio units, they run into all sorts of hardware and software related problems. Constant communication with the maker is essential to progress, and there are many cases where the design process drags on indefinitely. For example, when setting the built-in PLL, the designer has to know the specifications of the PLL itself, and be able to set the critical control timing. Transmit and receive control is also necessary, with cut-and-try timing control, and communication protocols must be created for coding the sent data and decoding the received data. With radio communication it is difficult to ensure synchronization between transmission and reception, and in addition, the designer must be familiar with methods such as introducing preambles for clock recovery and the like. Only after performing these operations can the designer start creating communication protocols for the local system. All these factors represent a very high threshold for typical users. While this might not present technical challenges for power users, in developing products, there is no time to be lost.
Command control radio units
In this field, Circuit Design has developed the MU-1, a low power radio modem, and a number of related products oriented towards sensor network and remote control applications. The MU-1 unit features a built-in 8-bit CPU and can be controlled by the user with readily available data transmission commands. Figure 2 provides a comparison of the design process using a conventional unit and the MU-1 module. The commands and data from the user controller are input via the UART interface, and the processes of the lower layer are determined by the command interpreter. Link processing and error checking is performed by the data link layer, and data addressed to the local station is identified. The physical layer automatically processes the data encoding and decoding, PLL settings, switching between sending and receiving and so on for the radio unit. Control of the MU-1 is performed entirely with 2-character commands, and communication is established by processing the responses to these commands and the received data responses. The command structure consists of communication parameter settings commands, UART parameter setting commands and so on. For example, the @CH command switches channels, whereas the @DI command specifies another device for communication. The command for actually sending data is the @DT command, and up to 255 bytes of data can be sent at one time. Total current consumption is kept to about 46mA (with a supply voltage of 3.3V when transmitting), including for the high frequency component.
Available interfaces
UART, which is commonly available with general purpose CPUs, is used for the interface between the MU-1 and the user controller. In the PC world, the COM port may seem like a legacy technology, but as it is simple and easy to use, it is still widely used for MCUs. The MU-1 can interface directly with the general purpose CPU UART, and to connect it to a PC, it only requires RS232 transceiver IC. CE approved, the MU-1 has a fast switching speed for a narrow band unit, with a wireless bit rate of up to 9,600bps.
System configuration
The MU-1 radio link uses a link ID, and the operation is performed automatically. The link related IDs are the User ID: UI, Group ID: GI, Equipment ID: EI, and Destination ID: DI. If all of these IDs do not match, data will not be output from the receiving MU-1. The Group ID is 8-bits long and up to 255 groups can be identified. The Equipment ID identifies the MU-1 and up to 254 units can be specified and arranged into one group. These IDs can be used for building highly flexible 1:1, 1:N or N:N systems. The MU1 can be configured with different interfaces for LAN, USB or RS232C, and Circuit Design has already implemented relevant board-based products, the MU1-LAN, MU1-USB and MU1-RS2, respectively (see figure 3). These can be used for evaluation purposes or for embedding in equipment. The MU-1 LAN interface board is equipped with Lantronics XPort with Ethernet support. This radio modem allows LAN-equipped factories to control remotely located equipment from a local PC. Since LAN cables can normally be extended about 100m, it is possible to locate the MU1-LAN in the best location, even in places that are distant from the main LAN. Although the data capacity of the MU1-LAN is not sufficient for data transmission between PCs, if it is used for LAN to LAN connections, the range of applications is further increased. The MU1-LAN is controlled through the COM port (RS232C). Currently many PCs do not have RS232C ports. But the MU1-USB radio modem can then interface any PC with a USB-port.
Development kits
The MU-1 evaluation kit MU1-EVK and the MU1-ADK are provided as evaluation and development support environments for MU-1 and its related products.An evaluation program including an «air monitor" software interface (figure 4) allows users to check the optimum unit configuration, at the design stage. The air monitoring function uses the received signal strength indication function of MU-1, displaying the level of the received signal as it changes over time.
