Abstract: Using the single-chip microcomputer as the main controller, the 220 V power line in the home is used as the information transmission medium, and the power carrier chip ST7538 is used to realize the signal acquisition and intelligent control of the home appliance. The software adopts C# programming and uses the personal computer in the home as the server. The website format allows users to log in remotely to form an intelligent control system for home appliances. Users can log in to the Internet at any time and any place, conveniently control the home appliances, and can query the various states of the home appliances at any time.
Power line communication has many advantages: long communication distance, no influence on terrain and landform, low investment, short construction period, simple equipment and low implementation cost. The power line network extends in all directions and covers all areas of urban and rural areas. It can fully utilize the existing low-voltage power line infrastructure, eliminating the need to re-erect lines, avoiding damage to public facilities and buildings due to wiring, and saving manpower and material resources. In this paper, the intelligent control system for home appliances is designed by using power carrier technology, so that users can log in to the Internet through various ways anytime and anywhere, and conveniently control the home appliances.
1 overall system design
The overall structure of the system is shown in Figure 1. The main controller uses AT89C52, a low-voltage, high-performance CMOS 8-bit microcontroller from Atmel.
Figure 1 System overall structure block diagram
ST7538 is a half-duplex, synchronous/asynchronous FSK modem chip designed by SGSTHOMSON based on power carrier chip ST7536 and ST7537 for power line network communication in home and industry. It is suitable for power carrier communication network applications. The ST7538 operates from a single supply, integrates a linear driver and a 5 V linear regulator, is controlled by internal registers, and is programmed using a synchronous serial interface; such as watchdog, clock output, output current voltage control, initial detection, The timeout mechanism uses BCDV technology to use DMOS and bipolar CMOS structures on the same chip.
The ST7538 has eight carrier frequencies, and only one frequency is available at the same time. The communication channel can be a combination of a normal mode or a multi-frequency mode. Different frequencies can be selected through the control registers, and the transmit and receive filters are also changed.
The ST7538 exchanges data with the host controller via a serial interface. When data is exchanged using RxD, TxD, CLR/T, the data transfer is managed by the REG_DATA and RxDx lines. There are four working modes: data transmission, data reception, read control register, and write control register, as listed in Table 1.
Table 1 Four working modes of ST7538
Line access has both asynchronous and synchronous communication modes, which can be selected by internal registers. When the ST7538 is in data receive mode, the internal phase-locked loop recovers the reference clock and the data on RxD is active on the CLR/T rising edge.
When the ST7538 is in data transmit mode, the reference clock is internally generated and the data on TxD is read on the rising edge of CLR/T. If the RxTx line is set to 1 and REG_DATA=0, then ST7538 enters the idle state and CLR/T is forced low. Over time, the modem begins to provide the received data on the RxD. If the RxTx line is set to 0 and REG_DATA = 0, the ST7538 enters an idle state and the transmit circuit is turned on. After a period of time, the demodulator begins transmitting data to the TxD port.
When reading and writing control registers, the ST7538 control registers are always in synchronous mode and use the same interface lines (RxD, TxD, and CLR/T) as the main interface and REG_DATA. When REG_DATA=0 and RxTx=0, the data of the TxD line is first written to the most significant bit (MSB) of the control register, and the TxD state is sampled on the rising edge of CLR/T. The contents of the control register are updated at the end of the register access (REG_DATA falling edge). If more than 24 bits are transmitted, only the last 24 bits are valid. When REG_DATA = 1 and RxTx = 1, the contents of the control register are transferred to the RxD port, stable at the rising edge of CLR/T, and the most significant bit (MSB) is transmitted first.
Figure 2 Power line interface circuit
When RxTx=1 and REG_DATA=0, the receiving circuit portion is in an active state. The input signal is taken from the RAI pin, referenced to SGND, and passed through bandpass prefilter (Â±10 kHz). The prefilter can be cancelled by setting the control register 23 bits. The input phase is in a wide dynamic range and receives very weak signals in noisy environments. The applied waveform amplitude is automatically adjusted by the automatic gain control mechanism (AGC) and then passed through a narrow-band bandpass filter, after which the signal is trimmed by a mixer through a sine wave generated by the FSK regulator before being sent to the FSK demodulator. The signal is further improved by a mid-band low-pass filter, and finally the FSK demodulator passes the signal to the RX logic for final digital filtering. When the RAI pin does not detect a flag or a null frequency, digital filtering eliminates noise signals with spikes that are much larger than zero. In receive mode, the flag and the null frequency must be pulled apart by at least "Baud Rate/2" for proper demodulation. When the ST7538 is in data receive mode, the transmitter circuitry (including the power interface) is turned off, which allows the device to achieve very low current consumption (typically 5 mA).
2 server construction
With a home computer, the Microsoft IIS server is installed on the computer and made into a website form, allowing users to remotely access their own pages. The page displays the current status of the home appliance, and can send a control command through a button to control the state of the home appliance, such as the light on and off. The webpage uses Visual Studio 2008 development software and is written in C# language. The host computer software and the lower computer main controller use the serial port for communication, and the master node controller and the slave node controller communicate on the 220 V power line through the ST7538.
3 power line interface circuit design
The function of the power line interface circuit is to couple the modem chip to the power line. Its performance determines the quality of the communication, is the key to achieve carrier communication, mainly including the transmission filter circuit, the receiving filter circuit and the coupling protection circuit. The transmission filter circuit filters the carrier signals output by ATOP1 and ATOP2, filters out harmonic noise and spurious signals doped in the signals, and couples the processed signals to the power lines with high efficiency and certain power. In Fig. 2, a fourth-order band-pass filter is formed by C4, L1 and C3, C5, R1, and L3, and the channel frequency used is 132.5 kHz; the receiving filter circuit is used to filter out unwanted signals and noises other than the specified frequency. The useful carrier signal is input to the ST7538, and the parallel resonant circuit is used. The second-order passive band-pass filter is composed of R2, L2, C2 and C1. The center frequency is 132.5 kHz (determined by the values â€‹â€‹of L2 and C2); the coupling protection circuit It is used to isolate AC 220 V strong electric signals, among which D1, D2 and D3 are used to prevent damage to high-level signals such as surges or shocks on the power line. The Vsense and CL pins of the ST7538 are used to introduce feedback signals for automatic gain control of the output voltage and current.
As can be seen from Figure 2, the ST7538 has relatively few peripheral components. The ST7538 is a highly integrated power carrier chip using FSK modulation technology. It integrates all functions of transmitting and receiving data, including power amplification and voltage/current automatic control, thus greatly simplifying the application circuit. The ST7538 also provides a watchdog, clock output, reset, 5 V/3.3 V power output, etc., which can be easily connected to a microcontroller.
In the power line interface circuit, the signal filtering part is a key part of the whole module, which includes an input narrowband filter and an output narrowband filter. The signal in the power line is transmitted to the left side of the transformer via a transformer coupling, and an input filter circuit is introduced from the junction of R1 and C5. The filtered signal of the parallel current resonant circuit composed of C2 and L2 is transmitted to the RAI port of the ST7538. Capacitor C1 is used to isolate the DC voltage that may be too high for protection. Figure 3 shows the input filter circuit.
As shown in FIG. 4, the output filter circuit is composed of C1, C2, R, and L in series. Its function is the same as the input filter circuit, which is an unwanted signal that allows signals of a specific frequency to pass and blocks other frequencies. This circuit is a series voltage resonant circuit. When the signal frequency in the circuit is exactly equal to its fixed resonant frequency, voltage resonance occurs in the capacitor and the inductor. The voltage in the capacitor is exactly equal to the voltage in the inductor, and the direction is opposite. At this time, the capacitive reactance and the inductive reactance are equal, the voltage and current in the circuit are in the same phase, and the circuit exhibits pure resistance. This phenomenon is called series resonance.
Figure 3 input filter circuit Figure 4 output filter circuit
4 single chip and ST7538 interface circuit design
The communication between ST7538 and MCU uses SPI interface. SPI (Serial Peripheral Interface) is a synchronous serial peripheral interface that allows a microcontroller to communicate with various peripheral devices in a serial manner to exchange information, as shown in Figure 5.
Figure 5 ST7538 and microcontroller interface circuit
The ST7538 can be asynchronously connected to the microcontroller using a three-wire interface (RxD, TxD, RxTx). Data exchange does not require any additional clocks and additional protocols. In the data receiving mode, the MCU must restore the reference clock and control the time for transmitting 1 bit in the transmit mode. When no carrier is detected, RxD is forced to pull low.
In addition, the ST7538 can also be connected to the microcontroller using a four-wire interface (RxD, TxD, RxTx, CLR/T). The ST7538 is always the master of communication and provides clock reference using CLR/T.
The interface between ST7538 and MCU is actually an SPI bus system, which can directly interface with a variety of standard peripheral devices produced by various manufacturers. The interface typically uses four lines: a serial clock line (SCK), a master input/slave output data line MISO, a master output/slave input data line MOSI, and an active low slave select line SS. In this circuit, CLR/T is equivalent to SCK, RxD is equivalent to MISO, TxD is equivalent to MOSI, RxTx is equivalent to SS, and UART/SPI pin is grounded. The data transmission and reception control uses the P2 port line. When RxTx=0, REG/DATA=0, the carrier chip is in the data transmission mode, and the TxD input is sent to the power line. When RxTx=1, REG/DATA=0, enter The data receives the data mode, the RxD outputs the data received from the power line; when REG/DATA=1, the read/write control register sets the chip function parameters. The synchronous clock signal of the CLR/T output data is connected to the external interrupt INT0 of the microcontroller, and the MCU reads and writes the ST7538 in an interrupted manner. P2.5 inputs the carrier sense signal. When ST7538 detects the carrier of the working frequency, CD_PD=0; when there is no carrier, CD_PD=1. In addition, the watchdog counter should be cleared within 1.5 s because the ST7538's watchdog time is 1.5 s.
Designed for home and industrial environments, the ST7538 is powerful, highly integrated, and employs a variety of anti-jamming technologies. Although it uses FSK modulation technology without the use of spread spectrum technology, it is because of its reliable communication in a wide channel environment with a wide noise band. If you can make good use of its multi-band nature, you can overcome the shortcomings of narrowband communication. The experiment proves that the system runs reliably and has high cost performance, which can be widely applied to smart home and intelligent building systems.
. ST7538 datasheet http://
. AT89C52 datasheet http://
. ST7536 datasheet http://
. ST7537 datasheet http://
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