Abstract: AVR microcontroller ATmega48 chip and CMX865 modulation / demodulation chip are used as the hardware architecture to standardize the external serial communication protocol. The CMX865 register is operated by the analog port line. A serial port to FSK communication module is designed to implement data transmission. Way of conversion. The hardware circuit uses the ATmega48 microprocessor as the core, and uses the C / BUS bus to control the CMX865 chip to realize the FSK / DTMF receiving and sending functions; the peripheral circuit is composed of ring detection and FSK data transceiver interface circuit. The software design of the module mainly includes two parts of serial communication and FSK communication. The serial communication program realizes the functions of packet sending and parsing / processing of serial communication commands; FSK communication program realizes the function of sending and receiving FSK data by interrupt mode.
With the rapid development of information technology and network technology, information interaction application business has brought more convenience to human life, and information terminals have shown great market potential. However, due to different terminal user environments, user information interaction methods may be different. Currently, IP methods, FSK methods, and wireless communication methods are commonly used. In order to improve the terminal market competitiveness and reduce the system development cost investment, the independent design of the communication module and terminal program is an ideal solution. This article uses the ATmega48 chip and the CMX865 chip to implement the FSK communication module. Based on this module, the FSK information interaction between the user and the business platform is a simple serial communication relative to the terminal.
1 Introduction to ATmega48
ATmega48 is a high-performance, low-power 8-bit CMOS microcontroller based on AVR enhanced RISC (reduced instruction set) structure. The microcontroller has a controllable power-on reset and programmable power-down detection circuit, a calibrated on-chip RC oscillator, 18 on-chip and off-chip interrupt sources, and 5 sleep modes. Due to its advanced instruction set and single clock cycle instruction execution time, ATmega48's data throughput rate is as high as 1 MIPs / MHz, and its operating speed is 10 times higher than that of ordinary single-chip microcomputers, thereby alleviating the contradiction between the power consumption and processing speed .
It integrates 4 KB of in-system programmable Flash, 256 bytes of EEPROM, and 512 bytes of SRAM on-chip. Its peripherals have a programmable serial USART interface and can work in the master / slave mode of SPI serial Interface; there are 2 8-bit timers / counters with independent prescaler and comparator functions and 1 16-bit timer / counter with prescaler, compare function and capture function, and a separate on-chip oscillator. Programming watchdog timer, etc. The hardware circuit of the ATmega48 / 88/168 chip can be fully compatible, and the AVR chip can be flexibly selected according to the actual needs of the software, which greatly facilitates the development and development of the system.
2 Introduction to CMX865
CMX865 is a new DTMF codec / decoder / FSK modem composite IC chip developed by CML. It uses a single high-speed serial bus control and is compatible with most serial interfaces. The main features of CMX865 are:
â—† Power supply voltage 2.7 ~ 5.5 V, low power consumption operation mode;
â—† Support V.23, Bell202 FSK 1200 bps encoding / decoding;
â—† Integrated high reliability DTMF, encoder / decoder;
â—† Integrated ring detection function, support voice detection function.
3 Hardware design
Figure 1 shows the serial to FSK communication module circuit. The system mainly uses the AVR microcontroller ATmega48 chip and CMX865 modulation / demodulation chip as the hardware architecture. The ATmega48 chip uses the C-BUS bus to control the CMX865 chip to achieve FSK communication. The IRQN terminal of the CMX865 chip is connected to the external interrupt 0 (INT0) of the CPU chip to ensure that the software can receive / send FSK data in an interrupt mode. The clock signal, chip select signal, response response data signal and receiving control data signal of the CMX865 chip are respectively connected to the ordinary I / O interface pins of the ATmega48 chip, and the CPU can control the CMX865 chip through the analog port line.
Figure 1 also provides the basic FSK receive / transmit data interface circuit and ringing detection circuit. In the FSK transmission circuit, the purpose of the external resistor at the TXA (15 pins) of the CMX865 chip is to match the AC impedance of the chip interface to the line. In the FSK receiving circuit, the capacitance between the CMX865 chip RXAFB (9-pin) and RXAN (10-pin) and the isolation transformer is designed to filter out high-frequency noise; by adjusting the two resistors R1 in the receiving circuit The resistance value of R2 can change the amplitude of the signal provided by the receiving end to the CMX865 chip. In the ringing detection circuit, IC1 is a photocoupler. When not ringing, the phototransistor is cut off, and RING is at a high level; when ringing, the ringing signal is stabilized by capacitive coupling and a voltage regulator, and the ringing voltage causes The light-emitting diode emits light, and the base of the phototransistor is irradiated, which causes the phototransistor to turn on. RING is at a low level, and a low-level ringing signal is detected through the ATmega48 pin.
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