Design and development of inverter power supply based on MT888O-DTMF

Dual tone multi frequency (DTMF) codec communication has the characteristics of strong anti-interference, low cost and long distance. This paper proposes an inverter power system based on DTMF remote communication. The structural characteristics of the DTMF transceiver controller MT8880 and the three-phase PWM generator SA8282 are introduced. The inverter power based on DTMF technology, which is composed of MT8880, single-chip microcomputer 80C51, SA8282 and IPM, has the function of low-cost and high-reliability remote data communication, forming a remote telemetry and remote control inverter power supply, and expanding the application range of the inverter power supply.

0 Introduction
In the remote measurement and control system (or remote multi-machine control system), dual tone multi frequency (DTMF) coding technology can be used to transmit low-cost data through the telephone network. DTMF is composed of a group of low-frequency signals and a group of high-frequency signals in a certain way. Each group of audio signals has 4 audio signals, and each combination has a high audio signal and a low audio signal, with a total of 16 combinations. It is mainly used for telephone dialing information transmission and has strong anti-interference ability. MT8880 is a communication controller dedicated chip that integrates DTMF type encoding and decoding. It has the function of sending and receiving, and can realize DTMF long-distance data transmission.
The inverter is the core part of the AC uninterruptible power supply (UPS) and AC variable frequency speed regulation system. It can independently constitute the system as a variable frequency power supply device. The new fully digital three-phase SPWM generator SA8282 launched by MITEL has a wide output frequency and high precision. It can interface with the microprocessor and complete the peripheral control function and protection function, which can realize the intelligent design of the system. The single-chip microcomputer, SA8282 and MT8880 chips are combined to design an inverter power supply based on DTMF technology. It has the function of low-cost and high-reliability remote data communication, forming a remote telemetry and remote control inverter power supply, which expands the application field of the inverter power supply.
This article introduces the design of an inverter power supply based on DTMF technology, which is composed of AT80C51 single-chip microcomputer, SA8282 waveform generator and MT8880 DTMF and AC-DC main circuit. SA8282 and MT8880 only require a small amount of peripheral hardware without complex software programming, making the system's circuit structure simple, low cost, easy to control, and the system stable and reliable.

1 Composition of the inverter power system based on MT8880-DTMF data remote communication
The inverter power system based on MT8880-DTMF data transmission is a remote telemetry and remote control system that transmits data through the telephone network. It consists of a front end and a back end. The front and back ends perform DTMF encoding, decoding, transmission and reception through the MT8880 controller, as shown in Figure 1. The front end is the inverter power supply and data transmission unit, and the back end is the main control display unit. The front end is based on the single-chip microcomputer 80C51 as the core, and the SA8282 waveform generator and the 7MBP75RA120 IPM power module are used as the main components of the inverter power supply. The DTMF encoding is performed by MT8880 and transmitted over a long distance via ordinary cables (telephone lines) to the back end MT8880 for receiving and decoding processing. The back end main control unit consists of a single-chip microcomputer 80C51, MT8880, buttons, display screen, etc. MT8880 decodes the received DTMF signal into BCD code and sends it to the single-chip microcomputer 80C51 for processing and display. The back-end main control unit transmits the command word to the front-end through MT8880, realizing duplex remote data transmission between the front-end and the back-end.

2 MT8880-DTMF Features and Transmit/Receive Design
According to the recommendations of CCITT, the encoding and decoding of DTMF is defined as shown in Table 1. Each digital signal is composed of two groups of frequency signals, a low-frequency group and a high-frequency group. It uses a method of taking two out of eight to form an audio signal. Due to the interference of false signals, it is widely used. The expression of DTMF digital signal is:

The two terms in the above formula represent the values ​​of the low and high tone groups, respectively, and A and B represent the sample value quantization baseline of the low tone group and the high tone group, respectively. When sending the standard frequency in Table 1 specified by CCITT, the frequency error of the DTMF signal shall not exceed 1.8%, and the signal limit duration of each digit shall be greater than 40ms, while the receiving device can reliably receive the deviation of 2%, and can normally receive the signal duration of 30-40ms.

2.1 Characteristics of MT8880
The MT8880C manufactured by MITEL using ISO-CMOS technology is a single-chip DTMS transceiver with a call processing filter, which has the characteristics of low power consumption and high stability. The internal structure of the MT8880C with integrated transceiver functions is shown in Figure 2, including a high-performance receiver with an internal amplifier with variable gain and a transmitter with a pulse counter. The receiving part adopts the industrial manufacturing standard of the DTMF signal receiver MT8870; the transmitting part adopts a switched capacitor for D/A conversion. The MT8880C has a standard microprocessor bus and directly interfaces with the MCS-51 series microcontroller. The internal registers of MT8880 include 1 status register, 2 data registers and 2 control registers. The internal registers provide a group mode. In the dual audio group mode, the DTMF signal is sent out according to the precise timing. MT8880 selects the internal call processing filter to call the audio signal through the microprocessor control. The pins are defined as IN+, IN-: op amp input; GS: Gain Slee-t, op amp output; REF: reference voltage; Vss power supply negative pole; OSC2: clock output; OSC2: DTMF clock/oscillator input, using 3.579545MHz clock; R/W: read and write control; CS: chip select; RSO: register select; CK: system clock input; IRQ/CP: interrupt request/call processing, sending interrupt request to CPU. When the call processing mode is selected, when there is a call signal on the input signal line, the output sends the corresponding mode signal; D0~D3: data line; Est: lag-before output. When a valid audio pair is detected, it becomes a high level, and immediately returns to a low level when the signal condition is not met; St/Gt: hysteresis output/protection output. When the voltage is higher than VTST, the detected audio pair is saved and the output latch content is updated at the same time. When the voltage is lower than VTST, the chip can receive a new audio pair again. The output of Gt affects the hysteresis time constant, and its state is a function of Est and the voltage applied to St; VDD: positive power supply.

2.2 MT8880 and MCU interface and receive/transmit design
The MT8880 and MCU 80C51 interface and receive/transmit design are shown in Figure 3. D0~D3 of MT8880 are connected to P1.0~P1.3 of 80C51, P1.4~P1.7 of 80C51 are connected to CK, RSO, CS and R/W of MT8880 respectively, and the interrupt request IRQ of MT8880 is connected to INT0 of 80C51.

When MT8880 is used as a DTMF receiver, the DTMF signal is sent to the IN+ and IN- terminals of MT8880, amplified by the operational amplifier and the dial tone frequency of the signal is filtered out, and then sent to two groups of sixth-order switched capacitor bandpass filters to separate the low-frequency group and high-frequency group signals. The frequency of the DTMF signal is detected by digital calculation, and translated into a 4-bit binary code according to Table 1 by the decoder, stored in the receive data register, and sent to the data bus D0~D3 when needed. At this time, the delay control flag b3 in the status register is reset, and the receive data register full flag b2 in the status register is reset. If MT8880 is set to interrupt working mode, then when the flag b3 is reset, IRQL changes from high level to low level, and an interrupt request is sent to the CPU. When the CPU responds to the interrupt and reads the data in the register, IRQL returns to high level.
When MT8880 is used as a DTMF transmitter, the 4-bit binary code on the data bus D0~D3 is locked in the transmission data register, and the frequency of the transmitted DTMF signal is generated by the frequency division of the 3.579545MHz crystal oscillator. The frequency divider separates 8 sine waves of different frequencies from the reference frequency. The row and column counters separate a high-frequency signal and a low-frequency signal in the eight-out-of-two manner according to the data in the transmission data register. The D/A conversion is performed by the switch capacitor, and the DTMF signal is synthesized in the adder and output from the TONE terminal.
Every time MT8880 receives an external signal IROL from high to low, IRQL is connected to the interrupt INT0 (P3.2) of 80C51. During the interrupt, the microcontroller reads the data D0~D3 from MT8880 into the internal data memory. After the interrupt service, IRQL changes from low to high and starts to receive the next signal (it should be noted that the interrupt time is less than the internal digital time interval of the dial when designing). When issuing a command to the outside, 80C51 transmits the internal data D0~D3 to the P0 port, and then transmits it from the P0 port to the D0~D3 of MT8880. The data is modulated by dual audio in MT8880 and outputs the DTMF signal from the TONE pin.
The OSC1 and OSC2 of MT8880 are connected to a 3.55MHz crystal oscillator, and the EST and St/Gt terminals are connected to an external RC integration circuit to delay the decoded data so that the CPU can read the data correctly.

3 Inverter power supply based on MT8880 remote communication and its main circuit design
The front-end system of the inverter power supply based on MT8880 data remote communication is shown in Figure 4. It consists of two major parts: voltage-type inverter power supply and DTMF MT8880 remote communication, including the interface between the microcontroller 80C51 and the LED and button, the interface between the 80C51 and the SA8282 waveform generator, the interface between the 80C51 and the DTMF communication controller MT8880, and the 7MBP75RA120 IPM power module controlled by SA8282 as the main circuit of the inverter power supply.

The voltage-type inverter power supply is divided into two parts: the main circuit and the controller. The main circuit adopts an AC/DC/AC power supply type inverter structure, which consists of a rectifier, an intermediate filter, an inverter and an isolation transformer. The input power stage adopts a simple and reliable three-phase bridge-type uncontrolled rectifier to rectify the AC power of the power grid into DC, and obtains a smooth DC voltage after filtering through the intermediate filter. The inverter switch uses three groups (six) of Fuji's 40KHz two-unit IGBT7MBP75RA120IPM power modules to form a three-phase H-bridge circuit. In order to achieve electrical isolation between the input and output of the inverter power supply and meet the requirements of the output voltage amplitude, a transformer must be included in the inverter power supply. For the three-phase variable frequency power supply, a transformer △/Y is connected to the output end for isolation and voltage transformation to reduce the size and weight of the power supply. It
consists of a single-chip microcomputer AT89C51, an SPWM generator SA8282, a driver HLA02B, a detection data acquisition circuit ADC0809, a protection circuit, a display circuit, etc., to complete the two major functions of control and drive output.

4 Design of inverter power system program and data transmission program based on MT8880
The inverter power front-end system program based on MT8880 data transmission includes the main program and the interrupt program. 80C51 realizes data exchange by initializing the internal registers of MT8880. The data exchange based on DTMF includes receiving and sending data transmission control, in which the main program completes the initialization, key monitoring and display parts. The interrupt program realizes data acquisition and data transmission, as shown in Figure 5.

4.1 MT8880 initialization procedure
During the system power-on, reset and control, the registers of MT8880 need to be initialized. There are five important registers in MT8880, including the receive data temporary register and the transmit data temporary register, the transceiver control registers CRA and CRB, and the transceiver status register, as shown in Figure 2. The receive data temporary register is used to store the valid data value received last time, and is a read-only register; the data in the transmit data temporary register determines the frequency composition of the dual-audio signal to be transmitted, and data can only be written to the transmit data temporary register; the transceiver control registers CRA and CRB occupy the same address and are used in turn. The operation on CRB is realized by setting a specific bit in CRA. The functions of CRA and CKB and the functions of the status register refer to the literature. During the initialization of MT8880, the microcontroller writes to the transmit data register when RSO=0 and R/W=0; reads the receive data register when RSO=0 and R/W=1; writes to the control register CRA/CRB when RSO=1 and R/W=0; and reads the status register when RSO=1 and R/W=1. The initialization procedure of the microcontroller for MT8880 is:

4.2 Data receiving and sending program design based on DTMF transmission
Data receiving program controlled by MCU for MT8880

5 Conclusion
MT8880 integrates the sending and receiving functions of DTMF signals, realizes the encoding, decoding and data transmission of DTMF signals, and greatly simplifies the complexity of the design. The simple and inexpensive DTMF method is used for data acquisition and remote transmission, which can be widely used in remote control and telemetry, such as a new device for automatically reading water meters, electricity meters, and gas meters in users' rooms from a long distance, as well as multiple automatic metering and charging devices for refueling at large gas stations and automatic weighing devices for large tower cranes. The
high-reliability DTMF encoding and decoding data transmission technology of MT8880 is combined with the inverter power supply controlled by the single-chip microcomputer to form a low-cost, high-reliability inverter power supply with remote data communication function, which expands the application scope of the inverter power supply and makes DTMF technology more widely used in the field of modern measurement and control. If the inverter power supply is used as a variable frequency power supply for the variable frequency speed regulation system of the AC motor, a variable frequency speed regulation system with remote data communication function is formed. Then, by simply changing the setting of the SA8282 initialization control word, the frequency and working voltage of the output AC power can be easily changed, saving a lot of programming work and achieving real-time control. Since the waveform is a pure sine wave, the harmonic impact is reduced and the work efficiency is improved.