【正文】 near control systems so long as the software design is reasonably changed. The system has been proved to be accurate, reliable and satisfied through field practice. KeyWords:AT89S51。 Microcontroller。 DS18B20。 Temperature I. Introduction Temperature is a very important parameter in human life. In the modern society, temperature control (TC) is not only used in industrial production, but also widely used in other fields. With the improvement of the life quality, we can find the TC appliance in hotels, factories and home as well. And the trend that TC will better serve the whole society, so it is of great significance to measure and control the temperature. Based on the AT89S51 and temperature sensor DS18B20, this system controls the condition temperature intelligently. The temperature can be set discretionarily within a certain range. The system can show the time on LCD, and save monitoring data。 and automatically control the temperature when the condition temperature exceeds the upper and lower limit value. By doing so it is to keep the temperature unchanged. The system is of high antijamming, high control precision and flexible design。 it also fits the rugged environment. It is mainly used in people39。s life to improve the quality of the work and life. It is also versatile, so that it can be convenient to extend the use of the system. So the design is of profound importance. The general design, hardware design and software design of the system are covered. Ⅱ. SYSTEM GENERAL DESIGN The hardware block diagram of the TC is shown in Fig. 1. The system hardware includes the microcontroller, temperature detection circuit, keyboard control circuit, clock circuit, Display, alarm, drive circuit and external RAM. Based on the AT89S51, the DS18B20 will transfer the temperature signal detected to digital signal. And the signal is sent to the microcontroller for processing. At last the temperature value is showed on the LCD 12232F. These steps are used to achieve the temperature detection. Using the keyboard interface chip HD7279 to set the temperature value, using the microcontroller to keep a certain temperature, and using the LCD to show the preset value for controlling the temperature. In addition, the clock chip DS1302 is used to show time and the external RAM 6264 is used to save the monitoring data. An alarm will be given by buzzer in time if the temperature exceeds the upper and lower limit value of the temperature. III. HARDWARE DESIGN A. Microcontroller The AT89S51 is a lowpower, highperformance CMOS 8bit microcontroller with 4K bytes of insystem programmable Flash memory. The device is manufactured using Atmel’s highdensity nonvolatile memory technology and is patible with the industrystandard 80C51 instruction set and pinout. The onchip Flash allows the program memory to be reprogrammed insystem or by a conventional nonvolatile memory programmer. By bining a versatile 8bit CPU with insystem programmable Flash on a monolithic chip, the Atmel AT89S51 is a powerful microcontroller which provides a highlyflexible and costeffective solution to many embedded control applications. Minimum system of the microcontroller is shown in Fig. 2. In order to save monitoring data, the 6264 is used as an external RAM. It is a static RAM chip, lowpower with 8K bytes memory. B. Temperature Detection Circuit The temperature sensor is the key part in the system. The Dallas DS18B20 is used, which supports the 1Wire bus interface, and the ONBOARD Patented is used internally. All the sensor parts and the converting circuit are integrated in integrated circuit like a transistor [1]. Its measure range is 55℃ ~125 ℃, and the precision between 10℃ ~85 ℃ is 177。℃ [2 ,3]. The temperature collected by the DS18B20 is transmitted in the 1Wire bus way, and this highly raises the system antijamming and makes it fit in situ temperature measurement of the rugged environment [4].There are two power supply ways for the DS18B20. The first is external power supply: the first pin of the DS18B20 is connected to the ground。 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 microcontroller 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 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 Dalla