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MT8880-based multi-function one-touch dial telephone system

In Electronic Infomation Category: M | on April 23,2011

With the changing market demand, the traditional phone functions gradually revealed a single, convenient dial-up calls and MAX1953EUB datasheet and other issues. This scheduling system for industrial areas and MAX1953EUB price and underground distress SOS project requirements, design a dial with one button, the remote setting, automatic answer hang up and MAX1953EUB suppliers and other functions of the new telephone system, the system can be used for both civilian areas, for the elderly, children, the disabled and other special groups and hotel customer service system provides a convenient solution.

1 system theory and composition

System block diagram shown in Figure 1, mainly by the master microprocessor, DTMF codec module MT8880 and external circuit, the external circuit including the ringing detection circuit, picking electromechanical Road, AGC automatic gain control circuitry, and audio amplifier.


Figure 1 System block diagram of the

PIC microcontroller as the system microprocessor controlled by the MT8880 work, the use of DTMF signal to achieve the one-touch dialing and remote settings. Ring through the ring detection circuit to receive signals by the microprocessor to determine the number of rings after achieving effective control of electromechanical analog hook pick, with the AGC circuit and amplifier circuit for audio smooth clear.

2 DTMF codec module

DTMF or dual tone multi-frequency, are easy to identify, anti-interference ability, the advantages of small amount of data transmission, the system uses the companys MT8880 chip MITEL decoding DTMF signals. MT8880 connected with the microcontroller data bus interface, can be directly controlled by the microcontroller, the internal registers contain 5 by pin RSI and R / W selection, MT8880 and MCU interfaces shown in Figure 2.


Figure 2 MT8880 and MCU interface circuit

DTMF signal by the TONE pin output, IN-pin input, R2/R1 determine the value of the internal receiver amplifier magnification, the higher the ratio the greater the receiver sensitivity. Resistor R4 and capacitor C2 value of the stability of receiving data, D0 ~ D3 data port output latch update time (R4 C2) is proportional to the value, if (R4 C2) value is too small will lead to DTMF After decoding the data signal jitter, resulting in receiving an error.

MT8880 with 6 modes of operation, the internal control registers CRA, CRB switch. When the same work in the DTMF transmission mode and burst mode, MT8880 sends continuous (51 2) ms of DTMF signals; when the work received in DTMF mode, by reading the internal status register SR MT8880 D2 bits determine the receive data is valid by the D0 ~ D3 into the decoded data; when the same work in the call processing mode and interrupt mode, MT8880 through the IN-pin detect a busy signal, by the IRQ / CP output pin for the same frequency square wave SCM identification, thereby automatically hang up.

3 peripheral circuit design

3.1 automatically pick Electrical Road Abstract Electrical and Mechanical Road

automatically detected by the ringing and analog circuit composed of two parts hook, the circuit shown in Figure 3. In accordance with the provisions of national standard GB3380, ringing signal voltage is 90 5 V sine wave, 1 s 4 s send-off, ringing through the blocking capacitor C12 into the bridge B1, by full-wave rectified by the VD3, VD4 the two regulators, and ultimately into a 12 V DC level rectifier drive optocoupler U2 work, microcontroller U2s work by recording the number of rings to detect. When the hook reaches the preset number of rings, the MCU control the work of optocoupler U3, resistors R25 to access phone hook loop, hook automatically.


ring detection and simulation in Figure 3 Summary electromechanical Road

3.2 Automatic Gain Control circuit

Good call quality, voice clarity is the phones basic requirements, this use of AGC automatic gain control to ensure smooth continuous speech signal.

AGC circuit can keep track, monitor, audio amplifier circuit output signal level and adjust the input signal. When the input signal increases, AGC circuit automatically reduces amplification gain; decreases when the input signal, AGC circuit amplification gain increases accordingly, so that the audio signal after amplification does not produce clipping and distortion. AGC circuit of the system by the multi-level attenuator, amplifier and feedback control circuit of 3 parts, the circuit shown in Figure 4.


automatic gain control circuit in Figure 4

Audio signal input through the blocking capacitor C6, the signal through R7, R8, R9 composed of three after the attenuator into the LM358 amplifier, magnification is 20 dB, amplified signal into the microphone of the speaker all the way, another way into the feedback control circuit, the feedback control circuit to amplify the signal detection rectifier as the input signal amplitude is proportional to the DC level, into the comparator and control multiple attenuator in order to achieve automatic gain adjustment.

In practice, for the frequency range 30 Hz ~ 20 kHz, the amplitude range of 200 mV ~ 1.5 V have good audio signal gain regulatory role to ensure the signal is not clipping distortion, amplified signal amplitude range should be between 0 ~ 6 V. Experimental waveforms shown in Figure 5, we can see that AGC circuit changes with the amplitude of the input signal conditioning circuit of the gain, the correction signal played a good role, part of the experimental data shown in Table 1.


Figure 5 AGG circuit waveforms

Table 1 AGG circuit experimental data


4 Software Design

MT8880 is Motorola68 interface bus with the use of microcontrollers, PIC microcontroller to read and write timing needed to achieve simulation MT8880 its control and communications. Read and write operations on the MT8880 has strict timing requirements, as shown in Figure 6, timing control register write MT8880, when the chip select line CS is set low selected MT8880, RS0, R / W set to write the appropriate register, the clock 2 valid data is written on each falling edge.


write control register Figure 6 MT8880 timing

MT8880 mode conversion is more complicated, each model is required before conversion to the internal controller registers properly initialized, the initialization register is cleared when the first CRA, and then select the CRB register and cleared as part of the program :


System is designed to pulse when the microcontroller receives a key, set the work in the MT8880 DTMF transmit mode, sending the two numbers before and after the gap of 0.5 s, to achieve a key dialing. When the system call, the 5 s before the MT8880 DTMF receive mode is set, then the system can be remotely calling telephone number and automatically receive earmarked to set the number of rings, set up to facilitate flexible, save manpower, data storage E2PROM in the PIC microcontroller in power-down will not be lost: 5 s after the MT8880 is set to call processing mode, to detect a busy signal and automatically hang up. Software flow chart shown in Figure 7.


software flow chart of Figure 7

5 Conclusion

The system is stable, reliable, efficient decoding, has broad application prospects, especially for coal mine scheduling, hotel customer service systems and the elderly. MT8880 call processing mode using the signal detection should pay attention to voice tone, ringback tones and interaction between busy, to determine the appropriate tone parameters can be detected effectively avoid interference. At present, the system has been applied to mine speaker phone system, users reflect the good.

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