Application of 51 single chip microcomputer in wireless data transmission

The general digital acquisition system converts the captured field signal into an electrical signal through a sensor, and after sampling, quantization and encoding by the analog/digital converter ADC, it becomes a digital signal, which is stored in a data storage device, or sent to a microprocessor, or sent to the receiving end for processing by wireless means. The wireless data transmission system is a set of equipment that uses wireless means to send the collected data from the measuring station to the main control station.

1 System composition

The system composition is shown in Figure 1 and Figure 2.

The system consists of two parts: the measuring station and the main control station. The measuring station mainly completes the collection and storage of field signals, receives remote control commands and sends data. The main work of the main control station is to send remote control commands, receive data information, perform data processing and data management, and randomly display and print.

2 Serial communication between AT89C51 and digital radio

Atmel's AT89C51 microcontroller is a low-power, high-performance 8-bit CMOS microcontroller with 4KB Flash ROM on-chip, an operating voltage range of 2.7 to 6V (actually uses +5V power supply), and an 8-bit data bus. It has a programmable full-duplex serial communication interface that can simultaneously transmit and receive serial data. It communicates with the outside world through the RXD pin (serial data receiving end) and the TXD pin (serial data transmitting end).

2.1 Communication Protocol and Baud Rate

The communication protocol between the digital radio and the microcontroller and terminal host is:

Communication interface - standard serial RS232 interface, 9-wire half-duplex mode;

Communication frame format: 1 start bit, 8 data bits, 1 programmable data bit, 1 stop bit;

Baud rate – 1200 baud.

The digital radio uses Motorola's GM series car radio, which works in the VHF/UHF frequency band. It can perform wireless data transmission (9-wire standard serial RS232 interface) and voice communication. It adopts binary frequency shift keying (2FSK) modulation and demodulation, which complies with the CCITT.23 standard of the International Telegraph and Telephone Consultative Committee. When performing digital transmission in the voice band, it is recommended to use a data rate of no more than 1200b/s. In actual use, the radio works in the frequency range of 220~240MHz, and the half-duplex mode (performing receiving and transmitting operations, but not at the same time) can meet the system requirements.

2.2 AT89C51 serial port working mode

The AT89C51 serial port can be set to four working modes, with 8-bit, 10-bit and 11-bit frame formats. In this system, the AT89C51 serial port works in mode 3, that is, an asynchronous communication format with 11 bits of frame: 1 start bit, 8 data bits (low bit first), 1 programmable data bit, and 1 stop bit.

Before sending, the software sets the 9th data bit (TB8) as the parity bit, writes the data to be sent into SBUF, and starts the sending process. The serial port can automatically take out TB8, load it into the position of the 9th data bit, and then send it out one by one. After sending, TI=1.

When receiving, set REN in SCON to 1 to allow reception. When a transition from "1" to "0" (start bit) is detected at the RXD (P3.0 end), start receiving 9 bits of data and send them to the shift register (9 bits). When RI=0 and SM2=0 or the received 9 bits of data are 1, the first 8 bits of data are sent to SBUF, the 9th bit of data is sent to RB8 in SCON, and RI is set to 1; otherwise, this reception is invalid and RI is not set.

Mode 3 baud rate = T1 overflow rate / n

When SMOD = 0, n = 32; when SMOD = 1, n = 16. The overflow rate of T1 depends on the counting rate of T1 (counting rate = fosc/12) and the initial value preset by TI.

Timer T1 is used as a baud rate generator and works in mode 2 (automatically reload the initial value). Assume that the initial value of TH1 and TL1 is X, then T1 will overflow once every "2 8-X" machine cycles. The initial value X is determined as follows:

X = 256-fosc × (SMOD + 1) / 384 × BTL

In this system, SMOD=0, wave rate BTL=1200, crystal oscillator fosc=6MHz, so the initial value X=F3H.

2.3 Hardware connection between AT89C51 and digital radio

The hardware connection between AT89C51 and digital radio is shown in Figure 3.

The system uses asynchronous serial communication to transmit measurement data. The microcontroller serial port is connected to the digital radio RS232 data port. The radio is in the receiving state (PPT=0, receiving state; PPT=1, sending state), and the microcontroller P3.5 pin outputs a high level. The microcontroller uses TTL level, and the radio uses RS232 level. MAX232 completes the conversion between TTL level and RS232 level. Three optocouplers 6N137 are used to isolate the power supply between the microcontroller and the radio, enhancing the anti-interference performance of the system.

The single-chip microcomputer controls the radio's transceiver conversion, command reception and data transmission through the three-state buffer gate 74HC125 with control terminal and the NOT gate 74HC14. When receiving, P3.5=1, c2=1, 74HC125B is cut off; P3.5 is inverted and photoelectrically isolated by 74HC14, making the radio's PPT pin low level and setting it to the receiving state; at the same time, c1=0, 74HC125A is turned on, and the received command is input from the radio's RXD terminal, and sent to the single-chip microcomputer RXD pin after MAX232 level conversion, photoelectric isolation, and 74HC125A buffer gate. When transmitting, P3.5=0, and the radio's PPT pin is at a high level after being inverted and optically isolated by 74HC14, setting it to the transmitting state; at the same time, c1=1, 74HC125A is cut off, c2=0, 74HC125B is turned on, and the data is output from the TXD pin of the microcontroller, and is sent out through the TXD port of the radio after being buffered by 74HC125B, optically isolated, and converted by MAX232.

3 Communication Software Design

Communication software is crucial. Once a problem occurs, the entire system will be paralyzed. It is very important to adopt error control and fault tolerance technology.

*The command sent by the master station contains a certain number of synchronization symbols 55H and a 3-byte password. The measuring station verifies the password after receiving 5 synchronization symbols in succession. After the verification is passed, the command byte is officially received; if it fails, the measuring station sends a signal to the master station to resend. If the verification fails three times, the command will be stopped. When the measuring station sends/the master station receives, the verification method is the same. After the verification is passed, the measuring station starts to send data.

*A command consists of 3 bytes, the second byte is equal to the first byte plus 35H, and the third byte is equal to the second byte plus 36H. If the received command does not meet this rule, the command will be resent, and the command will be stopped after three consecutive errors.

* Every time the master station sends a command, the measuring station sends back a response signal, which contains the original command sample.

The basic communication procedure between the microcontroller serial port and the radio is given below.

Initialization procedure:

BTL EQU 2FH; the baud rate is placed in the 2FH unit of the internal RAM

MOV TMOD, #21H; T0 mode 1, 16-bit counter, T1 mode 2, for serial port

SETB TR0 ; Start T0

MOV BTL, #0F3H; baud rate is set to 1200

MOV SCON, #0C0H; serial port mode 3, 9-bit data, reception disabled

Receiving and Verification Procedures:

NUM EQU 2BH; The synchronization symbol value is stored in the 2BH unit of the internal RAM

TEMP EQU 2CH

ROM-CH: DB 55H, 55H, 55H, 55H, 55H, 55H, 55H, 55H, 55H, 55H

DB 55H, 55H, 55H, 55H, 55H, 55H, 55H, 55H, 55H, 20-byte synchronization character

MIM DB 'WSC': 3-byte password "WSC"

SETB P3.5; Set the radio receiving status

SETB REN ; Allow serial port to receive

A1: MOV NUM, #0; record the number of consecutive synchronization characters 55H

A2: JB RI, A2; serial port has data to A3

A3: CLR RI; clear the receive interrupt flag

MOV A, SBUF; read serial port data

CJNE A, #55H, A1; not a synchronous symbol, transfer to A1

INC NUM; the number of synchronization symbols received plus 1

MOV A, NUM; Get the number of synchronization characters received

CJNE A, #5, A2; if you do not receive 5 consecutive 55H, you can switch to A2

A4:MOV NUM,#0; Password verification, record the number of password bytes received

A5: MOV DPTR, #MIM; password character first address

MOV A, NUM

MOVC A, @A+DPTR; look up the table to get the password

MOV TEMP, A ; save password

JB RI, A6; after receiving a byte, the serial port transfers to A6

…

A6: CLR RI; clear the receive interrupt flag

MOV A, SBUF; read serial port data

CJNE A, TEMP, A4; if the password does not match, transfer to A4

INC NUM; the number of passwords received plus 1

MOV A, NUM; Get the number of password bytes received

CJNE A, #3, A5; if the password is not received, transfer to A5

Sending procedure:

CLR P3.5; Set the radio transmission status

MOV B, #23

MOV DPTR, #ROM-CH

B1:CLR A

MOVC A, @A+DPTR; look up the table to send the synchronization symbol and password, a total of 24 bytes

INC DPTR

LCALL SEND-CH ; call the send single byte subroutine

DJNZ B, B1

…

CLR A

MOV DPTR, #7000H; external RAM data first address, send data in external RAM to the radio

B2: CJNE R4, #0, B3

CJNE R3, #0, B3; R4R3 = number of bytes sent

B3: MOVX A, @DPTR; get data

INC DPTR

LCALL SEND-CH

CJNE R3, #0, B4

CJNE R4, #0, B5

B4:DEC R3

LJMP B2

DEC R3

DEC R4

LJMP B2

…

SEND-CH: SETB TB8

MOV SBUF, A

DB 0, 0, 0, 0, 0, 0, 0, 0

JNB TI, $; delay 4 μs

CLR TI

RET

Conclusion

After the wireless data transmission system was built, it has been used for more than two years. The operation results show that the system works stably and reliably. The use of relatively complete software and hardware design and anti-interference measures ensures the safety and reliability of the system. The measurement station transmits the collected field signals to the main control station in a timely manner, which improves the real-time performance of data processing. The software and hardware design of the single-chip microcomputer and digital radio interface has strong applicability and can be widely used in wireless data transmission equipment.