Voice mailbox indication system design in the telephone

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At present, telecom value-added services in many places include the "voice mailbox" function. This feature has been integrated on some new types of terminal devices, such as push-to-talk phones, but most older phones do not have this feature, so customers have asked to develop a simple phone add-on to match these The phone implements voice mailbox indication.

Design requirements

● It can correctly distinguish the state of the hook machine (the parallel meaning is shown in Figure 1).

Figure 1 Switch, parallel, voice mailbox indicator box three connections

● The data of a specific location-specific letter in the serial EEPROM can be read out correctly.

● In the parallel on-hook state, you can receive caller ID of various standards and timings. According to the caller's number, the header part exactly matches the preset “voice mailbox prompt code” in the EEPROM (ie, if the mailbox is stored in the EEPROM prompt) If the code is 1234, then all the numbers starting with 1234 are matched, and an LED (ie, the voice mailbox indicator) is lit, and it flashes every 100 seconds. When you hang up, you should have a sleep state and turn on the line_det and IOC0 interrupt to wake up. If you need to illuminate the light, you can turn on the 2Hz interrupt to wake up, you don't need to light the light. For the sake of safety, the 32768 crystal oscillator is not closed. The received data does not have to be saved and will be cleared after the comparison is completed.

● It can correctly demodulate and dial the machine in the state of parallel pick-up. In other words, the system can correctly demodulate each dial when it is taken in parallel and save it to the cache for comparison. If the first digits of the dialed number match the preset “voice mailbox prompt code” in the EEPROM, the “voice mailbox indicator” is turned off and goes to sleep. If you can't match from a certain bit in the middle, you will immediately go to sleep, and turn off line_det and IOC0 to wake up and keep 2Hz wake up. In the 2Hz subroutine, judge whether to hang up. If you hang up, open line_det and IOC0 wake up. If the DTMF is not received within 10s after taking the machine, it will enter the same sleep mode.

● When the machine is in the state of sudden change, the CALL demodulation should be re-acquired. For example, if the system is in the CALL state and the state of the parallel machine changes abruptly, the system should be able to jump out of this CALL operation and clear the buffer area to receive the next CALL.

For reliability reasons, the data in the EEPROM should be read and compared when comparisons are needed. It is not recommended to read the cache first when powering up.

Among them, SPT6603A is a recently introduced microprocessor designed for caller ID and voice dialing products. Line_det and IOC0 are interrupt wake-up sources set by the SPT6603A microcontroller specifically for the Caller ID system. Line_det is used to detect the signal on the line, and IOC0 is used to detect the polarity inversion signal.

circuit analysis

The principle of the voice mailbox indicator box system is basically the same as that of the telephone system. The hardware circuit includes the Caller ID input loop, the parallel detection line, the impedance matching line, the lightning protection and polarity protection circuit, and the CPU control. Line, EEPROM, just the telephone system has more ringing lines, keyboard dialing lines, LCD display, handle call lines, hands-free calling lines, electronic switch lines and so on. Since the circuit of the telephone system is already a very mature circuit, it does not take much time to design each unit circuit in the hardware design. Only the parameters of the circuit and the indicator light can be adjusted to realize the voice mailbox indicator box on the hardware. Function, so the voice mailbox indicator box system can be simplified by the telephone system. The block diagram of the voice mailbox indicator box system is shown in Figure 2.

Figure 2 System block diagram

When the system is designed, the number stored in the EEPROM can be modified by a parallel machine. If it is not necessary to modify the pre-stored number by parallel, the DTMF dialing part can be omitted. If you need to modify the number stored in the EEPROM on the indicator box, you can add a keyboard to the system. The impedance matching circuit is mainly designed for some special switches. If the voice mailbox indicator box is connected in parallel with a telephone with caller ID function, this part of the circuit can also be ignored.

The principle and analysis of Caller ID input loop

The Caller ID can be understood according to the actual situation. If the Caller ID is used to modify the product or system, it is understood as Caller ID; when used to modify the signal, the caller identification information is understood. The schematic diagram of the Caller ID input loop and the CPU internal hardware decoder is shown in Figure 3.

Figure 3 Schematic diagram of Caller ID input loop and CPU internal hardware decoder

Signal flow: When the decoder is turned on, the OP in the figure is operated in a linear state by Enable, and the Caller ID signal on the line can be decoded by OP-C3-AGC.

Wake-up process: After the decoder is turned off, the OP in Figure 3 is disabled, and the comparator CMP connected in parallel with the input terminal obtains an upper low-low bias due to the presence of R1, R2, and R3, and the CMP outputs 0. Due to the existence of the polarity protection circuit on the line, there is always one pin at both ends of T and R in the state of low AC impedance, that is, the AC signal can be considered to be sent from one of the ends.

In the case of a ringing trigger, during ringing, the amplitude of the ringing is sufficient to reverse the CMP, and the CMP output is 1, and the program is turned on.

For direct transmission, for example, the voltage divided on R2 is 0.15V. If the peak value of the signal sent on T or R exceeds 0.15V, the signal can also overcome the bias voltage pre-applied to the CMP input. This causes a CMP inversion, which in turn wakes up the CPU for decoding.

In the case of a reverse polarity trigger, if it is inverted from T-R+ to T+R-, it corresponds to an increase in the T terminal and a decrease in the R terminal. After the liter is added to the input pin of the CMP, the CMP is reversed, so that the CPU can be aroused to receive CALL; if it is reversed from T+R- to T-R+, the T terminal is lowered and the R terminal is raised. Because the input pin corresponding to the T terminal itself is lower than the R terminal for CMP, this reverse polarity cannot cause the CMP to reverse. The circuit consisting of C5, R6, and R7 is set to solve this problem. When the T terminal is rapidly lowered, a negative pulse is generated on the RIN pin, which can wake up the CPU to receive CALL.

Principle and analysis of parallel detection lines

The significance of parallel detection is as follows.

● When the system receives CALL and judges the machine to pick up the on-hook state, it can know whether the received signal is from the switch or the parallel dialing;

● In the process of taking the machine in parallel, in order to save power, the machine should run a relatively low power mode;

● In order to implement the anti-theft function, the system must add parallel detection lines.

The implementation method of parallel detection: the DM pin is high in the state of parallel hang up, and the DM pin becomes low when the parallel machine is taken, the hopping system can accurately distinguish the parallel Take the hang state. It is worth noting that when the machine is taken off, the DM pin is always low regardless of the machine pick-up or hang-up, which means that the parallel detection is invalid when the machine is taken. The schematic diagram of the parallel detection circuit is shown in Figure 4.

Figure 4 Parallel detection circuit schematic

Conclusion

The entire hardware circuit that implements the voice mailbox indicator box function is tested. After the Caller ID is decoded by the on-chip hardware decoder of the SPT6603A, the signal processed by the hardware decoder is parsed by the writing software to obtain the decimal dial number that we want. The entire system is working properly.