Voice communication module circuit design using CMX638

　　CMX638 is a voice codec chip specially used in communication systems. This paper explains the basic structure and functions of CMX638, and gives the peripheral circuit design of the voice system based on CMX 638 and the software design and implementation of its communication controller. Practical applications show that this system can be applied to many communication occasions that require voice processing.

　　Voice communication system is a very important part of communication. How to transmit more high-quality voice information in a limited frequency band is a key research topic. Now, there are many voice processing chips, including CMX618/638 and AMBE2000. Compared with AMBE2000, although CMX638 does not have as many compressed voice rate options as AMBE2000, it contains a voice CODEC module inside, which can flexibly choose to use internal CODEC or external CODEC, so it is more attractive in practical applications. This article designs and implements a voice processing module based on CMX 638 .

　　Power supply circuit design

　　Since CMX638 contains analog circuits and digital circuits, its power supply also requires analog power supply AVDD, digital port power supply IOVDD and digital power supply VDD, where AVDD voltage is 3.3 V, IOVDD voltage is 3.3 V, and VDD voltage is 1.8 V. As shown in the figure, its power supply circuit uses chip LM1117-3.3, input voltage is 5 V, output voltage is 3.3 V, and IOVDD is used. Using the same circuit, just replace the chip with LM1117-1.8, you can achieve 1.8 V digital circuit power supply. The 3.3 V analog circuit power supply can be filtered by L1, C20, and C18 to reduce the interference between digital circuit power supply and analog circuit power supply. At the same time, the digital ground DGND and the analog ground AGND can be connected with a 0 ohm resistor. In the actual circuit design and PCB board production, in order to prevent electromagnetic interference, add a 0.1μF capacitor as close as possible to each power pin of the CMX 638 chip for filtering.

　　Voice circuit

　　The voice input circuit uses LM358 for amplification, as shown in the figure.

　　Since the internal CODEC of CMX638 is used, its 23-pin CSCEL needs to be connected to IOVDD, and the 15-pin SDI, 17-pin SCLK, and 18-pin STRB in the CODEC PORT interface need to be connected to DGND. In this state, since the internal analog circuit of CMX638 has its own input voice and output voice amplifier circuits, the microphone voice can be directly connected to the 5-pin (INPUTP) and 6-pin (INPUTN) of CMX 638 in a differential manner, and the output voice of its 10-pin (OUTP) and 11-pin (OUTN) is directly sent to the speaker. In actual applications, the two voice output pins cannot be short-circuited.

　　CMX 638 can be used in many communication environments, and it contains 16-bit A/D and D/A conversion, so there is no need for a separate codec module, and it can achieve multiple transmission rates and full-duplex data processing. After actual testing, the system can be applied to many communication systems that require voice processing.