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Based on low-power MCU Intelligent Integrated Instrument Design

Print View , by: iSee ,Total views: 19 ,Word Count: 1972 ,Date: Mon, 24 Aug 2009 Time: 4:03 AM

1 Introduction

Now, the world is from the industrialized, mechanized era into the information age. Instrumentation, as an information tool, plays an indispensable source of information role. As the sources of information must be accurate or less the maximum error, so in every modern instrumentation using a variety of forms of technology integration and synthesis, in the high-tech development of the information era, instrumentation is entirely a modern one of a combination of factors.

This article is designed based on low-power mcu Intelligent Integrated system is based on intelligent, digital and network-based next-generation intelligent instrument design concepts, a smart conditioning, smart buses, industrial networks, liquid crystal display, electronic storage technology, Comprehensive instruction instrument, adjust instruments, meters and record meter totalizer functions, measurement and control with high precision, high reliability, stability characteristics. Using high-brightness LED backlight 192 × 64 monochrome dot matrix LCD display with support for two-channel universal analog input, 1 channel analog output and 2-channel alarm output. This paper discusses the specific instrument of intelligent integrated software and hardware design.

2 Intelligent hardware block diagram of an integrated instrument

Instrumentation hardware architecture block diagram shown in Figure 1. Mainly by the power structure, its hardware, 24V power distribution output, universal input, analog output, alarm output, LCD interface, keypad interface, external memory interface, real-time clock, RS485 communication interface of the nine components.

Based on low-power MCU Intelligent Integrated Instrument Design

Figure 1 Block diagram of the hardware

3 Intelligent Integrated hardware detailed design of the instrument

Intelligent Integrated instruments to MSP430F149 as the primary processing chip. MSP430F149 is a low-power microcontroller, as the instrument is very suitable for mixed-signal processors.

3.1 universal signal input circuit

The input signal has voltage and current signals, as well as resistance signals, these signals are able to measure the input side, which we call "general-purpose input port", as shown in Figure 2. Voltage signal through the input1 and input3 input, IN1 voltage that is measured by measuring voltage signal. Current signal from input2 and input3 input, IN2 and IN3 by measuring the voltage, known resistance of the resistor R5, available current input value. Measurement PT100, PT100 with three-wire access. 3-wire-connection in order to eliminate the error caused by lead length, by the following formula may, based lead resistance r, then:

U1 = (R + r) × I (1)

U2 = (R +2 r) × I (2)

2 × (1) - (2), may

Based on low-power MCU Intelligent Integrated Instrument Design (3)

By equation (3) we can see the resistance PT100 measuring only with U1, U2, I related, thus eliminating the error caused by lead from the formula PT100 precision available from the U1, U2 of the measurement accuracy. Specific measurement methods are as follows: First, the measured voltage value U2, known 5V reference and resistance R4, may be considered as the amount of current flowing through the PT100, and then measured U1, the resistance can be calculated PT100.

Based on low-power MCU Intelligent Integrated Instrument Design

Figure 2 general-purpose input port

3.2 analog output and dot-matrix liquid crystal display design

Intelligent integrated instrument analog output is the industry-standard output 4-20mA. To get 4-20mA through the 1-5SV analog voltage output through the VI conversion circuits to be. Obtained 1-5V analog voltage output mostly through Digital to Analog (DAC) to do, but at present there is no integration of many of the internal microcontroller DAC (including the MSP430F149), even though some microcontroller integrated DAC, DAC's accuracy is often not high. In the high-precision applications still need an external DAC, so obviously increases costs. But almost all of the microcontroller (including the MSP430F149) provide timer or PWM output function. This instrument is the application of MSP430F149 of the PWM output, after a simple transformation of the circuit to achieve the DAC, which greatly reduce the AO part of the cost and reduce the volume and improve the precision.

Truly's LCD screen select MSC-G19264DYSY-070W STN screen, the screen pixels is 192 × 128, power supply voltage of 3.3V, in line with MSP430F149 of the I / O port level range, can be very easy to connect. 5V power supply backlight using high brightness than the D surface light design, so that the screen display, even in low visibility situations seem very bright, very clear. Its operating range of -20-70 ℃, the smart meter operating temperature range of integrated (0-55 ℃) within. The entire LCD screen operating current of only 75mA, than an ordinary dot matrix LCD screen to be much lower, thus greatly reducing overall system power consumption.

3.3 Real-Time Clock Design

Intelligent Integrated Instrument with real-time clock function, real-time clock chip PhilipsPCF8563, with high accuracy. With I2C bus interface, a few degrees up to 400KHZ. A year and a leap year tracking. Warning devices with a programmable, low-voltage monitor. Counter counts ranging from seconds to years, and counter / timer can be used to trigger precise timing applications. Power-on reset circuit within the belt. Operation of the clock with very low standby current at VDD = 3.0V and Tamb = 25 under the typical power consumption of only 250nA. Figure 3 is a PCF8563 application circuit.

Based on low-power MCU Intelligent Integrated Instrument Design

Figure 3 PCF8563 Application Circuit

3.4 The key interface with external memory design

Instrument panel, a total of eight buttons, namely, up, down, left, right, SET, ENTER, and two special function keys. Up, down, left, and right keys were used to screen move the cursor to the four directions, Upper and Lower also has a number of rolling function, SET button is used to activate the screen the appropriate option, ENTER key is used to confirm . Special function keys used to switch a screen and system configuration to run the display. Temporary retention of special function keys 2. Used in the design and the LCD screen to share data ports, through the 74HC245 to isolate. Because it has a 74HC245 tri-state output capability, so buttons and LCD screen does not interfere with each other.

Instrumentation provides logging capabilities, you can sample the data for each channel at intervals of a certain time interval (can be set) recorded. External storage data chips allow users to JMEL the company's serial flash AT25F2048. It consists of 3.3V power supply. The chip in terms of relative NANDFLASH with a simple interface to the characteristics of a three-wire SPI-interface, can be easily connected and the MSP430F149. And in capacity to meet the requirements, the costs should be significantly lower than NANDFLASH. It is relative to the E2PROM but also has large capacity, simple interface features. AT25F2048 capacity of 256K, a total is divided into four segments, 64K bytes. 256 bytes per page. It provides the hardware write protection and software write protection, support pages to write and byte write modes can be erased 10,000 times, consistent with this intelligent integrated instrument life.

3.5 RS485 Communications

RS485 bus as a multi-point differential data transmission electrical specifications, has become the most widely used one of the standard communication interface. RS485 allows a pair of twisted-pair on the multi-point two-way communication, it has the noise rejection capabilities, data transfer rate, cable length and reliability is unmatched by other standards. RS485 Protocol Specifications: Transfer Rate up to 10Mbit / s; maximum distance of 1200m; a maximum of 32 nodes: One group of twisted-pair cable, two-way master-slave communication; parallel connected nodes, multi-tasking communications.

Based on low-power MCU Intelligent Integrated Instrument Design

Figure 4 RS485 communication interface

RS485 data signal using differential transmission, also known as balanced transmission, it uses a pair of twisted-pair, the first line of which is defined as A, the other line is defined as B, under normal circumstances, sending drive A, B between the positively charged flat In the +2 V ~ +6 V, Negative ping-2V ~-6V. There is also a signal to C, in the RS-485 also includes an "enable" terminal. "So can" side is used to control sending the drive off with the transmission line and connection. When "enable" reach for the role, send the drive in a high-impedance state, which is different from the logic "1" and "0", to the state. Receiver and the transmitter is also for the relative provisions of income, the originator through a balanced twisted pair connected to the AA and BB counterparts, when in the closed end of AB is greater than +200 mV between the electricity normally, the output is logic level, less than -200mV, the output of negative logic levels. The receiver to receive a balanced line level range is usually between 200mV to 6V. System RS485 driver chip SP485, the circuit connection diagram shown in Figure 4.

4, the main instrument system software architecture design

System Software primarily by the system initialization module, data acquisition module, digital filtering module, data processing module, digital PID control module, LCD driver module, the system is running display module, the system configuration display module, key processing module, FLASH drive module, Data Remembrance module, real-time clock module, communication module. System software architecture Figure 5. For the detailed design of each module, due to space reasons this is no longer described in detail.

Based on low-power MCU Intelligent Integrated Instrument Design

Figure 5 system software architecture diagram

The author Innovation points:

This intelligent design from a functional point of an integrated instrument has the following characteristics: 1, instruments with a variety of functions, which set records meters, digital meters, Totalizer meters, regulating a variety of functions in one instrument; 2, instrument analog input and output with high accuracy, wide application; 3, instrument with a good real-time, to meet the needs of users of real-time; 4 instrument with a standardized bus interface.

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