Design and implementation of microwave radiation counting control unit based on 51 single chip microcomputer

Microwave radiometer is a microwave receiver that uses microwaves for remote sensing and thus detects ground objects. It is widely used in detecting the atmosphere, ocean, vegetation and soil. As an important part of the microwave radiometer, the data processing and control unit undertakes all the driving and control functions, and has very strict requirements on timing and accuracy. Due to the high reliability requirements of the system, a single-chip microcomputer is used as the core of the 220 GHz microwave radiation CNC unit. Through precise timing control, data acquisition, antenna control, state extraction, serial communication and other functions are realized. At the same time, the CNC unit has the characteristics of low power consumption, high sampling accuracy, and simple interface.

1 System structure
The block diagram of the microwave radiometer is shown in Figure 1. The analog signal output by the detector is used by the CNC unit to realize data acquisition and transmission, display and data processing, and control the system timing, power on and off, etc., which plays an important role in the design of the radiometer.

1.1 Hardware structure of numerical control unit
The system consists of a single-chip microcomputer, a high-speed A/D converter, a program memory ROM, a high-speed static RAM, a read-write control circuit, a communication circuit, etc., as shown in Figure 2. The address bus and the data bus are multiplexed by latches, and the read-write control circuit is
used to implement the RAM read-write control circuit. The numerical control unit uses Atmel's AT89C51 single-chip microcomputer as the controller, which has 4 KB flash ROM, 128 B RAM, supports 1 000 electrical erasures, and selects an operating frequency of 11.059 6 MHz. At the same time, AT28C256 is used to expand the program memory ROM to 256 KB, and IDT71256 is used to expand the data memory RAM to 256 KB, and time-sharing multiplexing is performed through the latch 54HC573.
The data acquisition part uses AD's 16-bit parallel analog-to-digital converter AD976A, and the sampling rate can reach 200 KSPS. The high-speed A/D uses charge redistribution technology for successive approximation analog-to-digital conversion, so there is no need for an external sample-and-hold device. The voltage input range is -10～+10V, with high resolution, and can achieve 16 bits without missing codes, meeting the precision requirements of microwave radiation CNC.
The AT89C51 serial port output is TTL level. In order to communicate with the standard RS 232 serial port of the PC, the system uses the MAX232 level conversion chip. [page]

1.2 Hardware Design and Implementation
1.2.1 Data Acquisition
After the microcontroller is initialized, the A/D converter is selected through the chip select signal, and the control of the RAM is handed over to the acquisition circuit. When AD976A performs data acquisition, the CS pin is fixed to a low level, and the conversion timing is controlled by the falling edge of the R/C signal, and the signal pulse width is at least 50 ns. When R/C becomes low, the BUSY signal also becomes low, marking the end of the conversion, and the data in the shift register is replaced by the updated binary complement. In the design, since the 256 KB data memory only requires 15-bit addresses A0~A14, A15 is used together with RD and WR as control lines to make A/D conversion and data storage work alternately. When A15 is low, the data memory IDT71256 is selected. At this time, CS is high, and AD976A stops working; when A15 is high, CS is low through the NAND logic circuit, AD976A is selected, and the R/C falling edge is provided through the change of WR, so that the sampling rate can be accurately controlled by software delay.
1.2.2 Bus control
In the hardware structure of the numerical control unit, the P0 port is used to provide the data bus and address bus. When the ALE output signal is high, the P0 port outputs the data latched into the lower 8 bits of the address in the bus driver, and at the same time, it forms a complete 16-bit address with the high 8 bits of the address sent by the P2 port, and addresses the external 256 KB address space. The A/D converter stores the collected data in the static RAM. Since the P0 port is a data and address time-sharing multiplexed port, the 74HC573 is introduced as the address latch. At the same time, RD is used as the control line for reading external data memory; WR is used as the control line for writing data to external memory; PSEN is used as the read control line for accessing external program memory.
1.2.3 Motor control circuit The
antenna drive control part sends the motor start command through the host computer. After the microcontroller receives the start command from the serial port, it sends a beat pulse to the motor driver AKS230 to drive the antenna to rotate at a uniform speed; the antenna speed can be controlled by programming the output beat pulse rate of the P1.0 port. When the host computer sends a stop command, the antenna state is extracted through the microcontroller program to ensure that the antenna starts and stops at the same position. Considering the matching problem of the microcontroller output current and the motor drive current III, the bus driver 74LS245 is used here to connect to the microcontroller, and the output current can reach 20 mA, which meets the motor drive requirements.
1.2.4 Serial communication circuit
The serial communication part sends and receives through the full-duplex serial communication interface RXD and TXD inside the AT89C51. The AT89C51 serial interface has four working modes. In this design, the serial port works in mode 1, the baud rate is variable, and the overflow rate is controlled by timer T1, so that T1 works in mode 2. Since the baud rate is (2SMOD/32)·(fOSC/12)·[1/(28-TH1)], the T1 load value can be calculated to be 0xFA, and the baud rate is 9600 b/s. Here, MAX232 is selected to complete the level conversion from TTL to EIA.

2 Software Design
The MCU software adopts a modular design method. It is divided into several parts according to the function, including data acquisition and storage, motor operation control and state extraction, and serial port transmission and reception. The modules are connected through interrupts or subroutine calls, and become an organic whole. The whole system has been debugged and successfully run using Medwin 3.0.
The data acquisition part includes initialization, signal acquisition and storage. The acquisition rate is controlled by programming to achieve a voltage value sampled every 10 ms and stored in the address space of 0100H~01C8H. After collecting 100 data, the interrupt is called to transmit to the host computer through the serial port for subsequent processing. Every time the system is powered on, the acquisition part is reset and runs from the beginning. The flow chart of the data acquisition part is shown in Figure 3.

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The motor operation control part receives serial port commands through interrupts to achieve precise start and stop of the motor, thereby controlling the scanning state of the antenna. When the serial port sends 5500h, the motor stops; when 55ffh is sent, the motor starts. The focus is on accurately reading the rotation angle state of the antenna to ensure that the motor starts and stops at the same position. It is mainly achieved through software counting. Since the motor step angle is 1.8° and 32 subdivisions are used, the number of steps per revolution is: 360°/(1.8°/32)=6400 steps. By reading the count value, the start and stop position of the motor can be controlled. The flow chart of the motor control part is shown in Figure 4.

The serial communication part adopts interrupt mode, including serial port initialization and serial port transmission. The interrupt type is judged by sending flag TI and receiving flag RI. If it is a receiving interrupt, the host computer command is received to control the start and stop of the motor; if it is a sending interrupt, 100 voltage values, i.e. 200 bytes, are sent to the host computer every 1 s. The flowchart of the serial communication part is shown in Figure 5.

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3 Upper computer software design
The upper computer software is mainly implemented through C#, which controls the lower computer and processes the data by calling the serial port, and can display the collected data waveform in real time and save it. The interface is shown in Figure 6

4 Conclusion
Single chip microcomputer is easy to control, with convenient interface and strong real-time performance. Based on these advantages, the microwave radiation CNC unit designed in this paper can not only accurately control the rotation state of the antenna, control the start and stop of the antenna, but also convert, collect, communicate and display the signal while the antenna receives external signals, and reflect the signal changes and the characteristics of the observed target in real time. The conversion accuracy is improved by using low-power 16-bit A/D, and the speed is adjustable, which fully meets the system requirements of microwave radiometer and has wide application in real-time observation.