This paper designs a building wireless visual intercom circuit, which uses the single-chip microcomputer as the controller core, and then connects the peripheral circuit to transmit the video signal through the wireless digital transmission circuit, which can reach the wireless transmission distance of 50-200 m. The advantage of this design is that on the basis of a single shot, a continuous display of the image is obtained, which truly realizes the wireless digital transmission of the image signal.
1 overall design
The system structure is mainly composed of three parts: the upper computer system, the lower computer system and the communication system. These three parts have completed the information exchange between the main controller and the sub-controller to achieve the purpose of building monitoring. The main controller and the sub-controller transmit data and commands through the wireless digital transmission module SRWF. The communication model belongs to a one-to-many communication mode, and its overall block diagram is shown in FIG.
The main control chip of the main controller is AT89C51, and the accessed room number is obtained through the keyboard. After confirmation, the MCU will control to open the video capture chip and the buffer chip to work, and communicate through the serial port under the set communication baud rate. The data is sent to the digital transmission circuit by the single chip microcomputer, and the digital transmission circuit transmits the data through the wireless channel. The master control chip of the controller also uses AT89C51 to receive the control information and data information of the master controller through the digital transmission circuit, and classify the received data. If the communication address signal sent by the host is received, the slave controller will read the vibration. The ring tone stores the data in the circuit, thereby emitting a ringing tone. Then, the video signal is received and displayed through the LCD display circuit. The user can determine whether to make further calls with the main controller through the button control circuit.
2 hardware circuit design
2.1 Circuit design of the main controller
The main controller circuit is composed of single chip AT89C51, keyboard scanning circuit, driving and digital display circuit, watchdog circuit, wireless data transmission circuit and so on. The hardware circuit schematic of the main controller system is shown in Figure 2.
The workflow is as follows: First, the user's key information is scanned through the keyboard scanning circuit to obtain the communication address of the slave, and the button information is displayed through 4 LEDs, so that the user can clearly know the slave address that he/she wants to access. The user can confirm the modification of the input value by adding value, depreciating, and re-entering on the keyboard. When the user presses the confirmation button, the host stores the slave address at this time, and starts the video chip sampling video signal, and simultaneously transmits the slave address through the wireless digital transmission chip SRWF. After receiving the response signal and ready signal of the slave, the host starts transmitting data at the set baud rate. Each frame of data is waiting for the response signal from the slave, and a check frame is sent for each frame of data to ensure the accuracy of the data transmission.
2.1.1 Keyboard scanning circuit
The keyboard scanning part adopts a 4×4 keyboard, and the scanning level is set through the P1 port. One of the lower 4 bits of the P1 port is set to zero first, and then the level of the P1 port is 4 bits. If the low position of the P1 port is zero, the column line is set low. If the high 4 bits of the P1 port detect that a bit is zero, the position of the button can be known through the position of the row and column, and there is a jitter problem during the button pressing. The method has hardware coding keyboard and software coding mode. In view of cost considerations and high performance consideration of AT89C51, the design adopts software coding mode to realize scanning. The keyboard scanning circuit is shown in Fig. 2.
2.1.2 LED display circuit
The LED display circuit is displayed in a dynamic display mode, and is composed of a 74LS248 BCD decoder chip and a 74LS138 decoder. The purpose of the 74LS248 is to save I/O port resources in order to control more peripheral chips. The segment selection data of the LED is determined by the segment selection code decoded by 74LS248, and the bit selection data is decoded by 74LS138. When working, the value of the button is first converted to the BCD code, and then sent to the 4th bit of the P0 port, but the overall copy of the P0 port will destroy the data of the bit selection port. At this time, it is necessary to correct the data of the P0 port, and the bit selection data is also sent to the 4th and 5th bits of the P0 port through the logical operation, and then the corrected data is sent to the P0 port, and the segment selection data is obtained at this time. There is also a selection of data. To make the displayed data not flash, you need to use the visual persistence of the human eye to control the time delay between each data display within 10 ms, so that the displayed numbers do not flash. The LED display circuit is shown in Figure 2.
2.1.3 Video acquisition and digital transmission circuit
The video acquisition circuit is composed of CAMERA 0V 7620 chip and IDT7205. This combination method is used because the amount of data acquired by the video is relatively large, but the operating frequency of the single chip microcomputer is relatively low. If they are directly matched, the two cannot receive normally. Therefore, the buffer chip IDT7205 is added, so that the operation of the MCU is controlled by the MCU to achieve the matching of the data amount and the MCU, and the IDT7205 is enabled only when the MCU allows reception. If the MCU monitors the VSYN of the video chip, it will output 1 at the P0.7 port, thus starting to control the IDT7205 to obtain data from the video chip. The SRWF module needs to set the wireless channel, interface type, interface rate, interface parameters, etc. before use. After the setting is completed, data transmission can be performed. When the module receives the first data sent from the MCU, it automatically performs wireless network. Connection and data synchronization work, so the first data will be output from the receiver serial port after a delay of 5 bytes, as shown in Figure 2.
2.2 Sub-controller circuit design
The sub-controller also uses AT89C51 single-chip microcomputer as the microprocessor. The peripheral interface circuit of the sub-controller system consists of crystal oscillator, LCD image display circuit, button unlock circuit, watchdog circuit, communication interface circuit and memory. The hardware circuit schematic of the sub-controller system is shown in Figure 3.
Its working process is: initialize its receiving baud rate, and it is always in the information monitoring state. When receiving the address frame, it is compared with its own address. If it is different, the frame data is discarded and the detection is continued. If they are the same, read the music code stored in the AT24C02 chip, and control the speaker to emit the music tone through the P2.1 port, thereby telling the user that a visitor requests to enter the building. When the user presses the answer button on the keyboard, the MCU will send the data received from the wireless digital transmission module to the P0 port for liquid crystal display. When the homeowner sees the image on the LCD, if the visitor is known, the button is unlocked; if the visitor is not known, press the hang up button to end the video call. The main working chip of the video display circuit is TFT6448B. TFT6448B has its own data latch. It does not need to expand the latch chip. After TFT6448B is selected, the data can be written to the corresponding row, column, control and data registers of TFT6448B to realize the control display of TFT6448B by AT89C51.
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