【正文】 besides not equals the preset value. Two: when the temperature is at somewhere between the two ends and equals the preset value, the motor stops. These steps are used to achieve the temperature control. In addition, the motor speed can also be adjusted by relative buttons. As shown in Fig. 8, the code data is input through ports A11~A8 (be ~) of the AT89C51 and inverted output by the inverter 74LS04. Finally it is amplified by the power amplifier 2803A to power the motor. SOFTWARE DESIGN According to the general design requirement and hardware circuit principle of the system, as well as the improvement of the program readability, transferability and the convenient debugging, the software design is modularized. The system flow mainly includes the following 8 steps: POST (Poweron selftest), system initiation, temperature detection, alarm handling, temperature control, clock chip DS18B20 operation, LCD and keyboard operation. The main program flow is shown in Fig. 9. Give a little analysis to the above 8 tasks, it is easy to find out that the last five tasks require the real time operation. But to the temperature detection it can be achieved with timer0 timing 1 second, that is to say temperature detection occurs per second. The system initiation includes global variable definition, RAM initiation, special function register initiation and peripheral equipment initiation. Global variable definition mainly finishes the interface definition of external interface chip connected to the AT89C51, and special definition of some memory units. RAM initiation mainly refers to RAM processing. For example when the system is electrified the time code will be stored in the internal unit address or the scintillation flag will be cleared. The special function register initiation includes loading the initial value of timer and opening the interrupt. For example, when the system is electrified the timer is initialized. The peripheral equipment initiation refers to set the initial value of peripheral equipment. For example, when the system is electrified, the LCD should be initialized, the startup display should be called, the temperature conversion mand should be issued firstly and the clock chip DS18B20 should also be initialized. The alarm handling is mainly the lowering and the raising of temperature to make the 安徽理工大學(xué)畢業(yè)設(shè)計(jì)外文文獻(xiàn)翻譯 12 temperature remain with the preset range. When the temperature is between the upper and the lower limit value, it goes to temperature control handling, that is to say the temperature need to be raised or lowered according to the preset value. By doing so make the condition temperature equal to the preset value and hence to reach the temperature target. 4 CONCLUSION The temperature control system has the advantages of friendly humanputer interaction interface, simple hardware, low cost, high temperature control precision (error in the range of 177。 the second pin serves as signal wire and the third is connected to the power. The second way is parasite power supply [5]. As the parasite power supply will lead to the plexity of the hardware circuit, the difficulty of the software control and the performance degradation of the chip, etc. But the DS18B20(s) can be connected to the I/O port of the micro controller in the external power supply way and it is more popular. Therefore the external power supply is used and the second pin is connected to the pin of the AT89S51. Actually, if there are multipoint to be detected, the DS18B20(s) can be connected to the 1Wire bus. But when the number is over 8, there is a concern to the driving and the more plex software design as well as the length of the 1Wire bus. Normally it is no more than 50m. To achieve distant control, the system can be designed in to a wireless one to break the length limit of the 1Wire bus [6]. C. LCD Circuit The LCD 12232F is used, which can be used to show characters, temperature value and time, and supply a friendly display interface. The 12232F is a LCD with 8192 12832 pixels Chinese character database and 128 168 pixels ASCII character set graphics. It mainly consists of row drive/column drive and 12832 full lattice LCD with the function of displaying graphics as well as 2 Chinese characters. It is in a parallel or serial mode to connect to external CPU [7]. In order to economize the hardware resource, the 12232F should be connected to the AT89S51 in serial mode with only 4 output ports used. The LCD grayscale can be changed by adjusting 安徽理工大學(xué)畢業(yè)設(shè)計(jì)外文文獻(xiàn)翻譯 10 the variable resistor connected the pin Vlcd of the LCD. CLK is used to transmit serial munication clock. SID is used to transmit serial data. CS is used to enable control the LCD. L+ is used to control the LCD backlight power. D. Clock Circuit The Dallas DS18B20 is used, which is a high performance, lowpower and realtime clock chip with RAM. The DS18B20 serves in the system with calendar clock and is used to monitor the time. The time data is read and processed by the AT89C51 and then displayed by the LCD. Also the time can be adjusted by the keyboard. The DS18B20 crystal oscillator is set at 32768Hz, and the remended pensation capacitance is 6pF. The oscillator frequency is lower, so it might be possible not to connect the capacitor, and this would not make a big difference to the time precision. The backup power supply can be connected to a rechargeable battery. E. Keyboard Control Circuit The keyboard interface in the system is driven by the HD7279A which has a +5V single power supply and which is connected to the keyboard and display without using any activedevice. According to the basic requirements