Based on the functional requirements of the automatic dishwasher [source: www.papersay.com, Paper Net], this paper introduces the design method of the hardware control system of the automatic dishwasher, which is designed by using the single-chip microcomputer AT 89 LS 8252 and the keyboard/display management chip HD 7279a. The hardware circuit of controlling keyboard and display with HD 7279 is given, and the flow chart of system software and functional software module is given. After testing and debugging, the system can effectively realize the automatic control of the dishwasher and complete the functions of automatic water intake, heating, cleaning and disinfection. 1 introduction With the popularity of digital information appliances, dishwashers have widely entered people's homes, thus gradually freeing people from complicated housework. Dishwasher, as a kind of home automation equipment, generally needs to realize the functions of automatic water control, automatic detergent addition, cold washing, automatic heating washing, cleaning, drainage, sterilization, timing and fault alarm. From the point of view of control quantity, dishwashers usually need to meet some specific requirements, as shown in figure 1, which involves two kinds of switching signals. One is to detect the high and low level signals of sensors, such as door switch, water level, temperature regulator protector, travel switch and so on. The other switch signal controls the disconnection and loading of strong and weak signals through solid-state relays, such as the opening and closing of water inlet solenoid valve, constant temperature heater, instantaneous heater, detergent control box and disinfectant control box. The analog signals involved in figure 1 are mainly used to realize the following functions: (1) The temperature sensor is used to detect the water temperature/disinfection water temperature; (2) Water temperature display, washing process display and fault type display are realized through man-machine interface display output, keyboard input and sound output; (3) According to the user's demand, the washing mode is selected by the setting key, and related functions are started by the function key; (4) Statistics of machine usage, statistics of detergent and disinfectant usage, etc. (5) Fault alarm, flushing completion report, etc. 2 Hardware design of dishwasher control system Aiming at the above functions, reliability, cost and convenience of use and maintenance, the author designed the dishwasher control system as shown in Figure 2. The minimum system of single chip microcomputer is designed by AT-MEL's low power consumption and high performance chip AT 89 LS 8252, which is compatible with MCS-5 1. On-chip 8 K bytes online programmable flash memory is used to store system programs, which can be downloaded through SPI interface. 2 K byte EEPROM is used to store key parameters, such as startup and maintenance password, product serial number, startup times, washing setting parameters, etc. 256 bytes of RAM are used to store temporary variables. In view of many on-off signals of this dishwasher, parallel chip 8255 is used to expand the I/O interface, and some pins of P 1 and P3 port are used together. The ratio of output to input is 2114, in which the buzzer is controlled by P 1.3 .. From the perspective of reliability, photoelectric isolation measures are adopted for the input and output parts. Considering that some driving signals need large driving current, ULN 2803 is selected as the power driver in this design. There are two schemes for temperature acquisition: one is to carry out AD conversion between thermistor and chip MAX 150, and then get the current temperature through the table stored in the program memory; Second, the three-terminal temperature sensor DS 18B20 is used to collect and directly obtain the temperature data. In contrast, the latter scheme is more economical and simple. Choose HD 7279a for keyboard and LED display? 3? For control, the circuit diagram is shown in Figure 3. Hd 7279a can directly drive 8-bit * * cathode LED digital tube without peripheral components; Pins Dig 0 ~ 3 correspond to 0 ~ 3 digital output drivers; Pins sa ~ SG and DP correspond to the driving output of a ~ g segment and decimal point; You can control the decoding/non-decoding, blanking and flicker properties independently. The chip has three types of instructions: the first is six 8-bit pure instructions without data, such as reset, test, left/right (cyclic) shift and so on. The second type is 7 16-bit instructions with data, including pressing 0/ 1 to decode and download data, downloading without decoding, blinking, blanking, segment lighting, closing and so on. The third is the instruction to read keyboard data. They all communicate with AT 89 LS 8252 in serial mode. The serial data is sent from the data pin to the chip and synchronized by the CLK terminal. When the chip select signal CS goes low, the data on the data pin will be written into the buffer register of HD 7279a at the rising edge of the CLK pin. In addition, the HD 7279a chip contains a debounce circuit, which can control the 64-key keyboard matrix. In fig. 3, the element jkey is a 4x4 keyboard interface. When a key is pressed, the key pin outputs a low level, and the Led 1 lights up, and this state remains until the key is released. At this time, if the command "read keyboard" 15h is received, in the first half of the command, the data pin will be in a high impedance state to accept the command from the microprocessor. In the second half of the instruction, the data pin changes from the input state to the output state to output the pressed key code value. 3 Software Design of Dishwasher Control System The function module of the system can be divided into six stages according to the dishwashing process: water intake process, heating process, detergent injection process, motor rotation control cleaning process, disinfection process by injecting disinfectant water and drainage process. At the same time, in order to ensure the normal operation of the dishwasher, the system also sets a fault alarm or stop function for the process that is prone to failure. The system software can be compiled according to the working flow of the dishwasher. The main software modules include power-on self-test, state initialization, displaying the current default state, entering the keyboard scanning state and waiting for the operation key to be pressed, and switching to the corresponding process according to the operation key. The main program flow is shown in Figure 4. Figure 3 Before using the dishwasher, the user should check whether it is in normal condition. If under normal circumstances, press the weak current switch, set the washing mode (or use the default mode) according to the washing requirements, put the bowl into the washing room, then close the machine door and press the start button. Then the controller detects the water level and water temperature to decide whether to supply water and heat at constant temperature. At this point, the preparation work is completed and the motor starts cleaning. Next, the cleaning agent is injected into the cleaning room and the water level is detected. If the water level is low, immediately inject water to ensure the cleaning effect. After cleaning, wait until the water on the bowl is empty for a period of time, and then inject the high-temperature disinfectant into the dishwasher for disinfection. After the washing is completed, the buzzer sounds to indicate the washing is completed, and the controller returns to the user's initial setting mode. Every process and key in washing corresponds to the turn-on and turn-off of a light-emitting diode to show whether the process and key are normal or not. After washing, the user opens the door, takes out the dishes, and then turns off the power to end the whole washing process. In the software subroutines of the system, it is necessary to explain the keyboard/display management module, including initialization subroutine, sending subroutine, receiving subroutine, interrupt subroutine, display subroutine and so on. Among them, the user can set the washing mode, the duration of each process and the amount of washing through the keyboard setting module, and can also press the corresponding function key to pause or stop the washing process at any time. Technical maintenance personnel can enter the correct password through the keyboard/display management module to inquire about the use status of the machine, such as machine serial number, machine use times, etc. You can also set key parameters according to the actual situation. Fault alarm function module can alarm with buzzer when working voltage fault, water inlet valve fault, heater fault, motor fault, detergent injection fault and high-temperature disinfection water injection fault, and at the same time cut off the high-voltage control signal to ensure system safety. Conclusion At present, the control system has been successfully debugged, and the test performance is good. It can realize the required functions of the automatic dishwasher, and it is simple and practical. However, there are still some areas that need improvement, such as manual intervention in drainage, better character or graphic LCD man-machine interface, and system integration needs to be improved.