1 Overview
In a dedicated communication system, teleconferencing is an indispensable function, such as the command and dispatch of troops, the dispatching of stations and stations, the tripartite conference of the telephone transfer station, and the conference recording. At present, the conference signal synthesizer processing special integrated circuit chips appearing on the market mainly include MC145611 of Motorla, MT8924 of Mitel, and M34116 of SGS-Thomson. M34116 has the following main technical characteristics:
* Can connect 1 ~ 64 party conference calls;
* Up to 29 group meetings;
* Can work in 3 modes at the same time, conference, transparent transmission and sound generation;
* Typical bit rate is 1356/1544/2048 / 4096Kb / s; 
* Compatible with all PCM frame formats;
* A / μ law is optional;
* Each channel has the same priority;
* There is a frame and time slot delay from sending to receiving;
* The gain of each channel can be controlled from 0 ～ 15dB;
* Sound production from 3.9MHz to 3938Hz, the minimum interval is 3.9Hz;
* There are 7 different sound outputs on the parallel port, up to 4 different continuous frequencies;
* A sound melody that can be programmed with up to 32 different continuous frequencies;
* 5V power supply;
* Compatible with TTL level input and TTL / CMOS level output.
2 Functional description
2.1 Pin description
M34116 is a 24-pin dual in-line package. The pins are shown in Figure 1.
2.2 Internal structure
When the system organizes a telephone conference, the output signal of any telephone interface participating in the conference must be sent to all the other telephones to receive the receiver. In other words, the signal received by the receiving end of each interface in the conference is the superposition of analog voice signals output from all other phone interfaces except itself. In the digital exchange, due to the binary nonlinear coding (A / μ law) characteristic of the voice signal, it is not allowed to simply add the binary signals from several channels. Figure 2 shows the internal structure of M34116. The conference chip must occupy a pair of bus bars. The PCM signal output from the switching network is input by the PCM I pin of the chip. After serial-parallel conversion, it is input to the linearization circuit. Each lead time slot (sample) after linearization plus the attenuation or gain level of the control system port is sent to the adder for addition, and the result after addition is linearized by POWER 10 and then sent to the frame RAM. Which time slots constitute a conference is written by the control system via the data bus mid-port. When an interface initiates a meeting, the control system assigns a number from 1 to 29 for the meeting. After that, join other existing conferences as needed, or reorganize another conference. The control system writes the number of each conference and the number of each time slot participating in each conference into the conference RAM through the data bus port, and the ground adder correctly combines and adds each signal. The signal output by the adder is restored to an A / μ-law encoded signal by a non-linearization circuit. Finally, a series of conversion circuits are restored to serial output and sent to the corresponding switched network PCM input bus. 
2.3 Function description
M34116 is a device specially designed for PCM digital switching applications, and can connect 64-party conferences and up to 29 groups of conferences. M34116 is connected to the switching network and the microprocessor as shown in Figure 3. The switching network can use digital switching networks such as M3488 or MT8980D. After PCM coding, its output time slot is delayed by one frame (plus one time slot) than the input time slot, so it should be delayed by one time slot when doing conference connection. The relationship between input and output is shown in Figure 4. M34116 has programmable attenuation or gain control for each channel of each working mode, and the input / output range is ± 15dB.
3 Control word software settings
The microprocessor controls the M34116 by setting the following command words. First set the operation mode, send the operation mode, set the instruction and select the function. Extension bit E = 1, F1, F0 set the PCM byte format. F1, F0 = 00 no bit flip, F1, F0 = 01 even bit flip, F1, F0 = 10 odd bit flip, F1, F0 = 11 all bit flip. M34116 must set the operation mode after reset.
3.1 Conference connection (Command 1)
The instruction consists of 5 bytes: the first 4 bytes are data bytes, 
The 5th byte is the control word. The content of the first byte is the conference group number and the conference start bit S. When S = 1, all registers of this group of conferences are cleared to 0. Only when the first slot of a new conference is connected, it is set. S is 1, P4 ～ P0 correspond to the conference group (1 ～ 29); the second byte is input attenuation or gain control, when AI4 = 1, it is gain control, when AI4 = 0, it is attenuation control, AI3 ～ AI0 Is the set value (0 ～ 15dB); the third byte is the output attenuation or gain control; the fourth byte is the time slot selection, when PT = 1, the PCM sampling signal before joining the conference is reversed and used to return Wave suppression; the 5th byte is the control word.
3.2 Transparent connection (Command 2)
This command is another function of M34116, which is the attenuation or gain level adjustment of PCM input and output of one time slot, delaying one frame and one time slot. This time slot is not a meeting participant.
3.3 Wire removal (Command 3)
Teardown is a meeting member, the end of the transparent connection or the end of the production, you must use this instruction to remove the line one by one.
3.4 Overflow information (command 4)
Single byte instruction, read conference overflow, and select B1 ～ B0 (0 ～ 3) by setting empty position. After this command, when the reading is valid, the data on the data bus is read according to the selected value to determine which meeting overflowed.
3.5 Sound production
Specify the time slot and sequence of sound for gain or attenuation. The sound sequence is composed of a maximum of 4 pairs of continuous sounds and a maximum of 32 pairs of continuous frequency melodies, with a frequency range of 3.9 Hz to 3938 Hz and a continuous range of 32 ms to 8610 ms. You can specify all time slots, or you can end with FFHEX bytes. M34116 continuously assigns or removes the thread. The melody can be programmed as a sine wave or square wave.
Up to 7 voices and 1 melody time slot can be generated at the same time. The command format for generating voice and melody is the same. The first byte is the output gain or attenuation, the second byte is the time slot number, the third byte is the operation code, the FFHEX byte is the end code, and Sn7 ～ Sn0 are the frequency intervals (n range is 1 ～ 4) ), Tn is the melody (n ranges from 1 to 32).
3.6 Status information
The status command is used to read the content of the command register or sound melody register, and read the status byte by specifying the time slot number. These bytes include: conference number or transparent mode or sound or not continuous, input / output attenuation or gain. If it is in sound mode, the continuous frequency sequence of sound is read. The first byte is the time slot selection, and the second byte is the control word. When the read time slot is set, the subsequent read value depends on the operation type (P4 ~ P0) of the sequence. P4 ～ P0 = 0, the disconnection state, the data read subsequently is invalid; P4 ～ P0 = 1 ～ 29, the meeting number; P4 ～ P0 = 30 voice or melody operation, the subsequent reading is the sound sequence; P4 ～ P0 = 31, for transparent connection.
4 M34116 typical connection circuit
The typical application circuit of M34116 is shown in Figure 5. The microprocessor is MCS-51 series single-chip microcomputer W77E58P, and the switching network adopts MT8980D of Mindy Corporation. Because the switching network uses a 4.096Mb / s clock, F0i is its synchronization signal (8kHz) input. After it is set low, the first falling edge of C4i starts the internal clock, and M34116 starts on the first falling edge of C4i. Therefore, when using the same clock, add a reverse circuit between the M34116 pin CLK and the C4i pin of the switching network. During a teleconference, the control system reliably needs to send some reminder tones to users participating in the conference. When an external circuit inputs a square wave with a frequency f at the TF terminal, the M34116 may output a sine wave PCM signal with a frequency f at the PCM OUT terminal; when TD is high, the chip outputs a signal tone and turns off the PCM voice signal . When TD is low, the output of the signal tone is prohibited, and the normal PCM output is restored. From which time slot the signal tone is output can be written in the appropriate control word specified by the microcontroller. This article only introduces the realization of the conference function, so the TD and TF pins are grounded. 
5 M34116 application examples in the private network
According to the previous introduction, through the interface circuit of Figure 5, the processor can read and write control words to complete the required functions. This article briefly lists the organization, joining, and disconnection of the conference. Other control words can refer to these use cases.
#define unch unsigned char
#define DATA34116 8001 // Data address
#define CTRL34116 8000 // Control address
#define START_CONF 0XFF // The meeting starts
#define JION_CONF 0X00 // Join the meeting
5.1 Organizing meetings
/ * Parameter ConfNumb: conference number (1 ~ 29). MeeTInGain: conference input gain. MeetOutGain: conference output gain. ChannelNumb: conference time slot. Flag: New meeting start flag * /
#pragma disable
void ConnetConf (unch ConfNumb, unch ChannelNumb, unch MeeTInGain, unch MeetOutGain, unch Flag) {
if (Flag == START_CONF) {
XBYTE [DATA34116] = 0x10 | ConfNumb; / * Conference number created * /
XBYTE [DATA34] = MeeTInGain; / * input gain * /
XBYTE [DATA34116] = MeetOutGain; / * Output gain * /
}
else if (Flag = JION_CONF) {
XBYTE [DATA34116] = 0x1F & ConfNumb; / * conference number * /
XBYTE [DATA34116] = MeeTInGain; / * input gain * /
XBYTE [DATA34116] = MeetOutGain; / * Output gain * /
}
XBYTE [DATA34116] = 0x40 | ChannelNumb; / * Conference time slot * /
XBYTE [CTRL34116 = 0x07; / * write control word * /
}
5.2 Disassembly
/ * Parameter ChannelNumb: conference time slot * /
#pragma disable
void DisconnetConf (unch ChannelNumb)
{
XBYTE [DATA34116] = 0x1F & ChannelNumb; / * Conference time slot * /
XBYTE [CTRL34116] = 0x0F; / * write control word * /
}
Figure 5
5.3 Tripartite meeting
The three parties A, B and C want to organize a meeting. Assume that the A bus is HW_A and the time slot is tsa; the B bus is HW_B and the time slot is tsb; the C bus is HW_C and the time slot is tsc; the input and output gains are all 0, and the HW_CONF is the M34116 bus.
(1) Conference initialization
ConnectConf (1,0,0,1, START_CONF);
// Create a conference, the conference number is 1, the time slot is 1
ConnectConf (1,0,0,2, JOIN_CONF); // Time slot 2 joins the meeting
ConnectConf (1,0,0,3, JOIN_CONF); // Time slot 3 joins the meeting
(2) Time slot exchange
connect (HWA, tsa, HW_CONF, 1); // Two-way connection of A's time slot and conference time slot 1
connect (HWB, tsb, HW_CONF, 2); // Refer to "Application of Microcontroller and Embedded System" 2002.7
connect (HWC, tsc, HW_CONF, 3); / or refer to the MT8980D data sheet
This constituted a tripartite meeting. You can also send the conference bus to HW_CONF and the time slot to 5 to a dedicated operating station to form a multi-mode or cross-mother conference. Note that the time slot must be 5. Because the output time slot is delayed by one frame and another time slot than the input time slot.
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