Application of DTMF special chip MT8880 in data transmission of communication network
Abstract: DTMF technology is widely used in communication networks. This article introduces how to use MT8880 this kind of DTMF special chip to realize simple data transmission between computers, which is characterized by using the I / O port of the single-chip microcomputer to generate the MT8880 interface signal timing. Using 89C51 as the controller, using MAX232 to achieve conversion, complete data communication between computers. The software and hardware design of the connection between MT8880 and 89C51, and between 89C51 and MAX232 are given.
Keywords: DTMF; data communication; single chip microcomputer; MT8880
Transmission of CommunicaTIon Network
GAO Xiu′e
(Department of Electronics & InformaTIon, Yangtze University, Jingzhou, 434023, China)
Abstract: DTMF technology is applied to communicaTIon network extensively. The paper introduced to use the kind of DTMFspecial-purpose chip base on MT8880 which receive and transmit the chip to realize the simple data transmission among the computer. It's characterisTIc utilizes a I / O to simulate interface signalofMT8880. Design and regard 89C51 as controller this time and make use of MAX232 to realize flatto change electricity, in order to finishing the data communications of computer. AT Meantme, MT8880 connects with 89C51, the hardware and software design of89C51 connects with MAX232 are given.
Keywords: DTMF; data communication; single-chip microcomputer; MT8880
Dual-tone multi-frequency signal (DTMF) is composed of a group of low-frequency signals and a group of high-frequency signals in a certain way, each group of audio signals has 4 audio signals, and each combination has a high-frequency signal and There are 16 combinations of low-frequency signals. In the past, it was mainly used for telephone dialing information transmission, and it has strong anti-interference ability. The DTMF special chips currently on the market integrate coding and decoding, and the MT8880 has a transceiver function. This article controls the MT8880 through the 89C51 single-chip microcomputer to realize the long-distance data transmission between computers.
1 DTMF technology introduction
1.1 Introduction to DTMF signal
The principle of DTMF signal generation: The dual-tone signal is the superposition of two sine wave signals. After selecting two frequencies f1 and f2, the mathematical expression of this signal can be obtained:
f (t) = A.sin (2.pi.f1.t) + A.sin (2.pi.f2.t)
If A / D conversion is performed on this signal with a suitable sampling frequency, it is easy to calculate the A / D value of each sampling point, and if these sampling values ​​are formed into a table, the same sampling frequency will be used in the microcontroller The values ​​in this table are output with a D / A converter, which is a dual audio signal. In practical applications, 1 b of DM coding is commonly used to implement the A / D and D / A processes. The A / D process can be completed on the PC. The program generates a DM coding table corresponding to each DTMF signal. The D / A process is in Completed on the microcontroller. Different from single-tone coding, DTMF signals are composed of two out of eight to form an audio signal, which is interfered by false signals, so the application range is particularly wide.
Comparing Table 1, it can be found that the DTMF signal is synthesized by two different frequency signals. Divided into high frequency group and low frequency group, the package contains 4 frequencies, which can constitute 16 different signals, corresponding to 16 different BCD codes in sequence.
1.2 Introduction to MT8880
MT8880C is a single-chip DTMF signal transceiver with a call processing filter. His manufacturing uses MITEL's low-power, high-stability ISO-CMOS technology. The receiving part of the DTMF signal adopts the industrial manufacturing standard of the DTMF signal receiving MCU MT8870; the sending part adopts the switched capacitor for D / A conversion to send the DTMF signal with high precision and low distortion. The internal register provides a group mode. In dual tone group mode, DTMF signals can be sent out with precise timing. The optional call processing filter allows a microprocessor to process the call audio signal. MT8880C also has a standard microprocessor general route directly connected with 6800 series microprocessors.
The structure of the MT8880C MCU that integrates the transceiver function includes a high-performance receiver with an internal amplifier with variable gain and a transmitter with a pulse counter. A standard microprocessor interface that can access registers inside MT8880. The internal registers of MT8880 include 1 status register, 2 data registers and 2 control registers, as shown in Figure 1.
2 System hardware design
2.1 System composition
The system consists of two parts: sending and receiving. as shown in picture 2. The communication steps between the two PCs are as follows:
(1) Connect the signal of the PC with the MAX232 of the serial port chip, and realize the level conversion through the MAX232 level signal in the PC.
(2) The signal converted from MAX232 is connected to 89C51 for processing, and the signal is introduced into the DTMF chip MT8880 for encoding. The encoded signal is connected to another MT8880 chip via an ordinary cable, which is performed by MT8880 Decoding processing.
(3) Connect the decoded signal to 89C51 for processing.
(4) The signal from the 89C51 passes through the serial chip MAX232 for level conversion again, and the converted signal is connected to the serial port of the computer to realize the communication between the two machines.
2. 2 MT8880 receiving circuit design
When MT8880 is used as the DTMF receiver, the DTMF signal is input through IN + and IN-, and the dial tone frequency in the signal is filtered through operation amplification, and then sent to the dual-tone filter to separate the low frequency group and high frequency group signals through digital The frequency of the DTMF signal is detected by counting, and it is translated into a 4-bit binary code by the decoder. The 4-bit binary code is latched in the receive data register, at this time the delay control identification bit in the status register is reset, and the received data register full flag bit in the status register is set. For external purposes, when the delay control identification bit in the register When resetting, IRQ / CP changes from high level to low level. If IRQ / CP is used as the interrupt signal of the microcontroller, IRQL changes from high level to low level, and an interrupt request is issued to the CPU. When the CPU responds to the interrupt and reads the data in the register, IRQ returns to high level. D0 ~ D3 of MT8880 are connected with P1.0 ~ P1.3 of the single chip microcomputer, P1.4-P1.7 are connected with CK, RSO, CS, R / W respectively, the circuit diagram is shown in Figure 3.
2.3 Design of transmitting circuit
When MT8880 is used as the transmitter, the D0 ~ D3 four-bit binary code on the data bus is locked in the transmit data register, and the frequency of the transmitted DTMF signal is generated by the frequency division of 3.58 MHz crystal. The frequency divider first separates eight sine waves of different frequencies from the reference frequency. The row and column counters separate a high-frequency signal and a low-frequency signal in eight out of two according to the data in the transmit data register, and do D / A through the switched capacitor After conversion, the DTMF signal is synthesized in the adder and output from the TONE terminal. The circuit diagram is shown in Figure 4.
3 System software design
3.1 Data transmission program design of MT8880
The corresponding serial port receiving program is used in the data sending program of MT8880, and its program flow chart is shown in Figure 5.
The list of procedures is as follows: 
3. 2 MT8880 data receiving program design
The corresponding serial port sending program is used in the data receiving program of MT8880, and its program flow chart is shown in Figure 6.
The list of procedures is as follows: 
4 Conclusion
MT8880 is used to make the DTMF signal receiving and sending functions in one, to complete the encoding and decoding of the DTMF signal, and to achieve the purpose of data transmission. It greatly simplifies the design complexity, can reduce costs and reduce volume.
references
[1] Principle and application of Sun Hanfang series single-chip microcomputer [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1988.
[2] Ma Jiachen SCM principle and interface technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1990.
[3] Xia Xuesheng. Computerized instrument design [M]. Beijing: Science and Technology Press, 1988.
[4] Wang Yi. Application manual of single chip device [M]. Beijing: People's Posts and Telecommunications Press, 1994.
[5] Gong Dongmei. Software design of data transmission interface based on DTMF signal [J]. Computer Development and Application, 2003, 16 (6): 17.
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