Applications M68HC908LJ12 single-phase multi-rate watt-hour meter designed to
With the rapid development of the national economy and people's rising living standards, electricity will be more striking. In 1995, China's full implementation of the TOU, by using the multi-rate watt-hour meter to facilitate the optimal allocation of power resources, stable supply order, multi-rate watt-hour meter performance and cost depends on microcontroller selection. M68HC908LJ12 U.S. Motorola Inc. launched in 2001, high-performance microcontroller, powerful for the development of multi-rate watt-hour meter, both to simplify circuit design, reduce costs, but also to improve the circuit's anti-jamming capability.
2 M68HC908LJ12 Microcontroller
M68HC08 series is a class of Motorola high-performance 8-bit microcontrollers with high speed and functional characteristics of strong and prices low. Hardware structure and M6805, M146805-level M68HC05 compatibility, but also with greater functionality. Its instruction set has more than 200 instructions and 16 different ways, the fastest instruction cycle is only 125ns, while the multiplication instructions are only 625ns. With a flash (Flash memory) M68HC908 series of microcontrollers, more greatly facilitate application development. M68HC908LJ12 is of a 10-bit A / D and LCD driver module microcontroller, its main features are:
· Maximum bus frequency 8MHz (4.5 ~ 5.5V), 4MHz (2.7 ~ 3.3V) and 1MHz (2.4 ~ 2.7V) internal bus frequency;
· 12K FLASH flash memory, with online programming and encryption functions;
· On-chip 512K bytes of RAM;
· Multifunction Timer interface TIM1 and TIM2;
· With infrared-enabled serial communication interface SCI;
· CPU work properly monitoring module COP;
· Supply voltage monitoring module LVI;
· Real-Time Clock RTC;
· Serial Peripheral Component extension interface SPI;
· 6 channel 10-bit A / D;
· 8 x Keyboard input interrupt.
Three single-phase multi-rate meter hardware components
Multi-rate table from the power measurement circuit, CPU, LCD display, real-time clock, communication interface, power, etc. components. Hardware block diagram of Figure 1.
3.1 Energy Measurement part of the
At present, the energy metering some of the general use of a dedicated power measurement IC. Which the United States AD's AD7755, CS5460 and other high cost, using the IC, the power measurement accuracy, reliability and production debugging, school table is assured. The system uses the AD7755, metering pulses generated by the metering chip, M68HC908LJ12 for measuring pulse-counting and in accordance with the current rates be treated accordingly.
3.2 LCD display part of the
Multi-rate watt-hour meter display generally use the LCD liquid crystal display, M68HC908LJ12 with LCD driver inside the module, thus saving plug-in LCD driver chips. M68HC908LJ12 segment LCD driver module, the number of choice for the 26 4,27 3,27 1. Dynamic drive, 1 / 3 bias.
3.3 Real-Time Clock section
Time is measured in time-sharing basis for energy. Current applications are more widely clock chip PCF8583, DS1302, 4553 and so on. This design uses the DS1302 is the United States introduced a serial interface DALLAS dedicated real-time clock chip. Chip has a programmable clock and calendar 31 bytes of RAM, a wide operating voltage range of 2.5V ~ 5.5V, low power consumption, the power-down after the battery can work for 10 years.
3.4 Processor part
Processor part is M68HC908LJ12, this microcontroller series of microcontrollers and other circuitry to achieve compared to the external expansion of the circuit very small. Watt-hour meter needs to power-down without loss of the contents of EEPROM data memory in the past tends to be used to achieve, M68HC908LJ12 a 12K FLASH flash memory, with online programming and encryption capabilities can be used as storage of such data. Meter always encounter power failure, you needed a brown-out detection, the general plug-in voltage detection chip, while the M68HC908LJ12 there LVI (low voltage detection module), enables the detection of the power supply voltage, resulting in a reset or interrupt can be set. Watt-hour meter with infrared remote control circuit, in order to achieve fast meter reading and parameter setting, in the past this part of the circuitry required to implement special infrared chip modem, M68HC908LJ12 infrared modem functions can be directly driven serial interface SCI infrared transmitter and receiver diodes. M68HC908LJ12 there KBI keyboard interrupt key functions can be easily achieved.
3.5 Communication Interface part of the
In order to achieve and the meter reading and automatic meter reading data transfer between systems, in the design of the RS485 communication interface part of the interface and infrared communication interface. Of which 485 interface circuit using MAXIM3085. Infrared communication interface module as M68HC908LJ12 there are infrared serial interface SCI, saving plug-in infrared modem circuit.
4 single-phase multi-rate meter software design
Single-phase multi-rate form of software is mainly to achieve power data acquisition and processing, and through infrared and 485 interface with the meter reading system for data exchange. Figure 2 shows the system block diagram of the main program.
The main module has the following functions:
· Power processing module's function is to provide metering pulses from the metering chip, CPU on the measurement of pulse count, combined with the current hours and rates, accumulated user peak, flat, valley of all periods of actual consumption.
· Communication module's function is achieved in accordance with communication protocol and the meter reading device and the meter reading system of reliable communications. Electricity by meter reading system to read user data and settings hours, rates, address, and other meter parameters.
· Display module function is to show the user's peak, flat, valley, the total electricity consumption and time, last month, electricity and other information.
5 Anti-interference Design
Watt-hour meter system, the interference source is mainly power grid voltage fluctuations, lightning, and the impact of electromagnetic fields generated by transformers and so on. Is well known that the higher the degree of integration, the fewer discrete components, the circuit more compact circuit connection, the stronger anti-interference ability. As can be seen from the above greatest feature of this design is the integration - in addition to measurement, the clock, 485 parts, the rest of the functional requirements M68HC908LJ12-chip module can be completed. For the power of the sudden power-down and a sudden power-up will produce a fast transient pulse group phenomenon, the design in the power into the line at using the magnetic ring, in the CPU power supply-side addition of decoupling capacitors. For the moment generated by high voltage lightning strike, this design uses a high-voltage varistor to absorb the impact of an instant. In addition, the system take full advantage of M68HC908LJ12 chip COP module (hardware watchdog), to monitor CPU's work, in the program into a "death cycle", the generated reset.
In taking the above-mentioned anti-interference measures, while the hardware, software, anti-jamming technology, can not be ignored, for pulse measurement and statistics to determine, in general, digital filter to avoid miscarriage of justice. For the procedure "to go fly," using software redundancy and "watchdog" technology. Rewritten for critical data protection, can be used to determine the conditions attached. For the keys to shake, using duplicate detection method. Of course, software and hardware anti-jamming technology, anti-jamming technology is not independent of each other, they are mutually reinforcing.
This design takes advantage of the internal function modules, reducing the external circuit, reducing costs, while increasing the reliability of the machine, the design of energy meter in the market is very competitive.
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