【正文】 serve as accurate twoway mouth of I/O in mon use. Expand among the system of memory outside having slice, P2 mouth see high 8 address off。 P0 mouth is a twoway bus, send the introduction of 8 low addresses and data / export in timesharing Output grade , P3 of mouth , P1 of P1 , connect with inside have load resistance of drawing , every one of they can drive 4 Model LS TTL load to output. As while inputting the mouth, any TTL or NMOS circuit can drive P1 of 8051 onechip puters as P3 mouth in a normal way . Because draw resistance on output grade of them have, can open a way collector too or drainsource resistance is it urge to open a way, do not need to have the resistance of drawing outerly . Mouths are all accurate twoway mouths too. When the conduct is input, must write the corresponding port latch with 1 first . As to 80C51 onechip puter, port can only offer milliampere of output electric currents, is it output mouth go when urging one ordinary basing of transistor to regard as, should contact a resistance among the port and transistor base , in order to the electricity while restraining the high level from exporting P1~P3 Being restored to the throne is the operation of initializing of an onechip puter. Its main function is to turn PC into 0000H initially , make the onechip puter begin to hold the conduct procedure from unit 0000H. Except that the ones that enter the system are initialized normally,as because procedure operate it make mistakes or operate there aren39。t mistake, in order to extricate oneself from a predicament , need to be pressed and restored to the throne the key restarting too. It is an input end which is restored to the throne the signal in 8051 China RST pin. Restore to the throne signal high level effective , should sustain 24 shake cycle (namely 2 machine cycles ) the above its effective times. If 6 of frequency of utilization brilliant to shake, restore to the throne signal duration should exceed 4 delicate to finish restoring to the throne and operating. Produce the logic picture of circuit which is restored to the throne the signal:Restore to the throne the circuit and include two parts outside in the chip entirely. Outside that circuit produce to restore to the throne signal (RST ) hand over to Schmitt39。s trigger, restore to the throne circuit sample to output , Schmitt of trigger constantly in each S5P2 , machine of cycle in having one more , then just got and restored to the throne and operated the necessary signal insidly. Restore to the throne resistance of circuit generally, electric capacity parameter suitable for 6 brilliant to shake, can is it restore to the throne signal high level duration greater than 2 machine cycles to guarantee. Being restored to the throne in the circuit is simple, its function is very important. Pieces of onechip puter system could normal running,should first check it can restore to the throne not succeeding. Checking and can pop one39。s head and monitor the pin with the oscillograph tentatively, push and is restored to the throne the key, the wave form that observes and has enough range is exported (instantaneous), can also through is it restore to the throne circuit group holding value carry on the experiment to change. At present,MCU to infiltrate all areas of our lives, which is almost difficult to find traces of the field without SCM. Missile navigation equipment, aircraft, all types of instrument control, puter network munications and data transmission, industrial automation, realtime process control and data processing, extensive use of various smart IC card, civilian luxury car security system, video recorder, camera, fully automatic washing machine control, and programcontrolled toys, electronic pet, etc., which are inseparable from the microcontroller. Not to mention the area of robot control, intelligent instruments, medical equipment was. Therefore, the MCU learning, development and application of the large number of puter applications and intelligent control of the scientists, engineers. SCM is widely used in instruments and meters, household appliances, medical equipment, aerospace, specialized equipment, intelligent management and process control fields, roughly divided into the following several areas:（1）In the application of Intelligent Instruments SCM has a small size, low power consumption, controlling function, expansion flexibility, the advantages of miniaturization and ease of use, widely used instrument, bining different types of sensors can be realized Zhuru voltage, power, frequency, humidity, temperature, flow, speed, thickness, angle, length, hardness, elemental, physical pressure measurement. SCM makes use of digital instruments, intelligence, miniaturization, and functionality than electronic or digital circuits more powerful. Such as precision measuring equipment (power meter, oscilloscope, various analytical instrument).（2）In the industrial control application With the MCU can constitute a variety of control systems, data acquisition system. Such as factory assembly line of intelligent control,all kinds of alarm systems,and puter networks constitute a secondary control system.（3）In the Appliance of Household appliances It can be said that the appliances are basically using SCM, praise from the electric rice, washing machines, refrigerators, air conditioners, color TV, and other audio video equipment, to the electronic weighing equipment, varied, and omnipresent.（4）In the field of puter networks and munications applications MCU general with modern munication interface, can be easy with the puter data munication, networking and munications in puter applications between devices had excellent material conditions, are basically all munication equipment to achieve a controlled by MCU from mobile phone, telephone, miniprogramcontrolled switchboards, building automated munications call system, train radio munication, to the daily work can be seen everywh