The Application of New Modem Chip CMX868
US Consumer Microcircuit Limited MX-COM Inc. The new product CMX868 developed by the company is a new multi-mode modem chip. Can be applied to low-voltage V. The design of 22bis modem. The function of CMX868 chip has not been fully developed in China because there are few relevant introductions. This article introduces the design scheme of developing a set of communication equipment by using single chip microcomputer and the modem chip and related hardware. Because this design does not have very high requirements on the performance of the terminal (only need to send or receive data between 0 and 1999 to the outside from time to time), it is constructed by a single-chip computer (such as AT89C51) that is less than one-tenth the price of the PC. A terminal system that can control the behavior of the modem chip can meet the requirements, and can also greatly reduce the operating cost of the device. 
1 Working principle of the system
1.1 CMX868 main chip
CMX868 multimode modem is a low voltage V. The modem chip of 22bis is very suitable for embedded systems. It uses low geometry CMOS technology, the normal operating voltage is 2.7V, the operating current is 5A, and the power consumption in the same state is zero. CMX868 supports V. 22, Bell 212A? V. 23? Bell 2002? V. For protocols such as 21, the maximum transmission rate is 2,400 bps. Setting up its internal parameter register allows the chip to communicate with the outside world in a specific protocol. But this setting will affect the signal modulation and demodulation method, and also determines the signaling system used in the communication process.
1.2 Signal transmission
The transmission path of the CMX868 data signal in the system is shown in Figure 1. Among them, CMX868 and the one-chip computer are connected through the serial port, the chip is often in the "zero state" waiting mode, the current consumption is only 1μA. The ring signal can be used to activate the CMX868 in zero state to make it enter the working state. The CMX868 chip can generate an interrupt signal IRQN to notify the MCU to allow it to receive input data, and then through the P1.0 pin of the MCU (connected to the CSN on the CMX868) and the T1 pin (connected to the SERIAL CLOCK on the CMX868) to CMX868 sends the signal. When they are in the "enabled" state at the same time, data and addresses can be transmitted through the two data lines COMAND DATA and REPLY DATA respectively.
figure 2
The previous design usually connected the ring detection circuit and the two-wire interface circuit to the CMX88 and the telephone line, respectively. 
This design integrates the two into a whole, which simplifies the circuit and improves the reliability of the system.
2 Hardware design
Figure 2 shows a Modem system circuit based on CMX868 designed by the author. This circuit design refers to the relevant design principles and ideas abroad [1] ~ [4], and also takes into account the domestic component source and Other factors.
The design of the printed circuit board can be completed by the software protel99se, and the network table, layout, and wiring can be imported after the principle design is completed, thus completing the production of the printed circuit board.
3 software design
The main task of the system is to set the initialization of the CMX868 chip and the parameters of the communication line. At the same time, the collected information is encoded and sent, and the received information is decoded and sent to the display system. The main program flow of the system can be divided into "information sending" and "information receiving" two relatively independent parts. Figures 3 and 4 are the software flow charts of the system's data sending and receiving, respectively, and the program can be written in C language. Since the data receiving and sending process is constantly switching, when it is in "receiving information" 
In the state, since the data amount of the received information is not known, the program may be required to make a judgment on the received information in order to switch the mode after the end of the preset information is received. In addition, the initialization of CMX868 and the parameter setting of the communication line are only necessary when the system is powered on and started, so there is no need to repeat it during mode conversion.
4 Test results
The debugging of this system is carried out between "source" (system A) and "sink" (system B). When debugging, first complete the circuit characteristic test of the two systems and the joint debugging of software and hardware, and then debug the communication process between the two systems A and B. When the number 1234 is entered on the keyboard of the system A, the data displayed by the digital tube of the system B coincides with it, thus confirming that the developed system can meet the design requirements.
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