ZigBee technology is a kind of short-range, low complexity, low power consumption, low data rate, low-cost two-way wireless communication technology, which is mainly applied to the field of automatic control and remote control. It is developed to meet the wireless networking and control of small and cheap equipment. ZigBee is a global standard for wireless connection, which focuses on reliability, low cost, long battery life and easy application. It is not only a simple alternative to cable interconnection, but also can establish a wireless network between different ZigBee devices, which is convenient to realize data communication and exchange. At the same time, it has the characteristics of low power consumption, high data transmission rate and low cost. Using the low power wireless transmission and ad hoc network function of ZigBee technology, we can realize “isolated monitoring” and “dynamic monitoring”. ZigBee technology combined with LAN and Internet technology can also realize “remote monitoring” and “home health monitoring”, which is an ideal choice to realize wireless ward call service.
The advantage of this scheme is that the hardware platform only needs a very small number of external components and equipment, the cost of the corresponding components is low, and the platform can run stably. Because the portable terminal is powered by battery, and low power consumption is also the key to the success of ZigBee system, so low power consumption is also the goal of this system design. In order to reduce the power consumption of the system to the greatest extent, from the perspective of hardware design, we use the MSP430 series processor of TI company, which is famous for its low power consumption, and the CC2420 chip of Ma level is used as the wireless transceiver chip. The combination of the two hardware platforms performs very well in power consumption. From the perspective of software design, we can also make good use of msp340f4618 According to the power saving mode of the chip, watchdog, interrupt and timer are used to wake up the device into the normal working state. The rest of the time, the device is in the sleep state of almost no power consumption to reduce the system power consumption.
Hardware design of system call module
According to the actual application requirements, we design the overall structure of the circuit, which can be divided into several parts: MSP430 module, RS232 module, display module, sound and light alarm and keyboard module. Chipon CC2420 is used for RF chip, MSP430FG4618 is used for microcontroller, key is used for input device, led and LCD chip are used for output device, RS232 and JTAG are used for external interface, which can meet the needs and support function expansion. CC2420 and MSP430FG4618 are battery powered. The block diagram of hardware structure is shown in Figure 3-1.
Figure 3-1 overall hardware structure
MSP430 module is the core of the whole system. The call module designed in this paper is composed of MSP430FG4618 and peripheral circuits. Figure 3-2 shows the core circuit of MSP430FG4618, including JATG interface, reset circuit, power supply and clock circuit. At the same time, the unused chip pins are led out to the edge of the circuit board, which is convenient for expansion when needed in the future.
Figure 3-2 MSP430FG4618 minimum system circuit diagram
In order to be able to carry out serial debugging, coordinator node and host are connected through RS232 interface, RS232 can provide a variety of low-speed communication at different rates. RS232 adopts negative logic, that is, logic “1” is – 5V ~ 15V, logic “0” is + 5V ~ + 15V, and noise tolerance is 2V. Because RS232 level is not compatible with MCU level, level conversion chip MAX232 is needed to convert RS232 negative logic level into TTL positive logic level. The circuit design is shown in Figure 3-3.
Figure 3-3 RS232 interface circuit