Design and Implementation of Digital Frequency Meter Based on SCM. Weiqiang Zheng

Transcription

1 Applied Mechanics and Materials Submitted: ISS: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Design and Implementation of Digital Frequency Meter Based on SCM Weiqiang Zheng Lanzhou University of Arts and Science, Lanzhou, Gansu, China Keywords: High precision, digital, frequency meter, AT89C51 SCM. Abstract. The digital frequency meter was a device to detect and display the frequency. In many design complexity, features a variety of electronic devices, the digital frequency meter was one of the essential equipment. It was widely used in various fields. The core of the frequency meter is AT89C51 microcontroller. It used SCM counting and timing functions to achieve the measurement pulse within a certain gate time, and it uses LED digital display tube displays the measured frequency. The device has a high frequency accuracy measurement, real time, and it has a simple, easy to carry, strong expansion capability, a wide range. Introduction In electronic technology, the frequency is one of the most basic parameters. It is the number of electrical parameters of the measurement program; the measurement results have a very close relationship, so the frequency of measurement becomes more important [1]. The SCM digital frequency meter with high reliability, small size, low price, features congruent, are widely used in a variety of intelligent instruments. These intelligent instrument control operations during instrument verification and measurement process, to achieve automation, switches and knobs on the instrument panel is replaced by a traditional keyboard, testers at the time measuring just press the desired key, save a lot of tedious the manual adjustment, intelligent instruments usually can automatically select the range, automatic calibration [2]. Some also automatically adjust the test point, so that not only facilitate the operation, but also improve the test accuracy. This paper presents a frequency meter based on AT89C51 microcontroller [3]. The Basic Principles of Digital Frequency Meter Digital frequency meter is a measure of signal frequency devices to decimal numbers to be displayed. It can measure the frequency sine, square, triangle wave signals. The basic principle of the frequency meter is to use a frequency source with high frequency stability as the reference clock to measure the frequency of other signals by contrast. Under normal circumstances, calculate the number of pulses per second, in which case we call the gate time 1 second. Gate time can also be greater than or less than one second. The longer the gate time is, the more accurate the resulting frequency values are but the longer the gate time is the longer intervals once every measurement frequency are. The shorter the gate time is, the faster the frequency values measured refresh, but the accuracy of the measured frequency will be affected. The digital frequency meter is the instrument using the digital to display the frequency of the signal measured, the signal measured can be a sine wave, square wave or other signals with cyclical changes [4]. The so-called frequency is that the times of the change of the periodic signal in a unit time (1s). If the number of repetitions of the change of the periodic signal measured in a certain time interval T is, the frequency can be expressed as: f = / T. Therefore, the block diagram of the digital frequency meter measuring frequency is shown in Fig. 1. Among them, the shaping change the measured signal into a pulse signal, the repetition frequency is equal to the measured frequency fx. Time reference signal generator provides a standard pulse signal of time, and cycle is 1s. The gate signal is controlled by the standard second signal when the second signal came, the gate opened, the measured All rights reserved. o part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,12:37:58)

2 Applied Mechanics and Materials Vols pulse signal is sent to counter decode display through the gate. At the end of the second signal, gate is closed, the counter stops counting. When using the counter method to achieve frequency measurement, the external test signal is a count source with timer and counter, using the delay program of the software to achieve counting gate. When using the timing method to achieve frequency measurement, the external test signal changes into square wave whose width is equal to the measured signal cycle through the preprocessing of frequency counter, square wave also added to the timer / counter input pin. The working process of this frequency meter is that: First timer / counter count register is cleared, and then detect whether the high level square wave added to the timer / counter input pin, when the high added to determine timer / counter input pin, running TR control bit, set 1, start the timer / counter, count for the machine cycle of SCM,and detect whether a square wave high end or not; when the end of the high TR cleared, stop counting, then read out measurement data from the counter deposit period, after the completion of data processing by the display displays the measurement results. Measured signal Shaping Pulse signal Gate Counter Decoder Frequency display Gating signal Time reference signal Fig. 1 Frequency Block Diagram The Overall Design of SCM Frequency Meter Using SCM to design to achieve frequency meter, compared with the use of a large number of digital devices designed frequency meter uses fewer components, its theory is simple and it is easy to debug and it is cost-effective. The block diagram is shown in Fig. 2. Signal amplifying Signal shaping AT89C51 SCM Digital frequency display Fig. 2 Microcontroller frequency meter The frequency meter choose 51 series microcontroller AT89C51 as the control to achieve the function such as signal control of the, display output data processing and control operation to complete a variety of digital timing logic control, counting. The working processes of frequency meter is that firstly, amplifies the measured signal by amplification, and send the measured signal to the waveform shaping, making the measured sine wave or square wave triangle square wave. Use the function of the counter and timer of SCM to count the measured signal. Write a program to prepare the corresponding program allows the microcontroller to automatically adjust the measurement range and the measured frequency data to the display display. System Key Technologies System Software Design. The development tool of this design is Keil C52. Keil C52 is 52 series and compatible microcontroller C language software development system produced by Keil Software Corporation in the United States, using traditional C language syntax to the development, compared with the assembler, C language function, structured, readable, maintainable there are the obvious advantages of greatly improving the efficiency and project development cycle, it can be embedded assembler, can be embedded in a critical position, so close to the compilation of the program to achieve efficiency [5]. The main flowchart design is shown in Fig. 3. The timer / counter are first set

3 3468 Manufacturing Technology, Electronics, Computer and Information Technology Applications to work the counter register is cleared, and the home run control bit TR is a start to treat count measured signal. Counting gate delays by the software program, starting from the minimum gate count, that is, from the beginning of the measurement frequency of the high range. TR gate at the end of the count is cleared, stop counting. Count registers value through channel 10 hexadecimal decimal conversion program to convert to decimal. Highest digit decimal numbers to discriminate, if the bit is not 0, the effective number of bits of data to meet measurement requirements, together with the measured value and range information to the display module; If this bit is 0, the counting gate width expansion 10 times, re-treat technology detection signal until the effective number of bits of data to meet the measurement requirements. By using the 12 MHz crystal oscillator, the timing of a second has exceeded the maximum timer value available. To achieve the second timing, the timing and counting may be used a combination of methods. Selection of timer / counter T0 make timers work in the way a generation timing of 50 ms, then the software counts the way it counts 20 times, you can get the timing of a second. Signal Amplification and Shaping Circuit Design. Because the microcontroller can only count on the pulse wave, but the actual measurement signal frequency needs are diverse, there is a pulse wave; there may be a sine wave, triangle wave, so they need a. The test signal can be converted into a pulse wave to be counted. Because the threshold voltage of the affected subject to temperature, supply voltage and disturbances, poor stability with Schmidt trigger AD gate constituted. The 555 timer is relatively high sensitivity, large output drive, and Schmidt trigger, and a simple structure consisting of a 555 timer, and the anti-interference ability is stronger than Schmidt trigger constituted with AD gate. The 555 Schmidt shaping is shown in Fig. 4. The Time Base Signal Circuit Design. The design of the time base composed of a crystal oscillator and divider. This uses 32768HZ crystal oscillator, using CD4060 chip after 14 minutes to get a signal frequency of 2Hz (32768/214), then after 74HC74 dual D flip-flop after two minutes to get 0.5Hz frequency square wave pulse signal so that the output seconds SCM counted. The principle is shown in Fig. 5. Start Scanning get started? T1 and T0 initialization To 1S? The counter stops counting Displayed frequency value End The counter starts counting The timer starts timing Store count results Converted to a decimal number Fig. 3 Fequency meter main flowchart RV1 47k C6 47µF R13 39k R1 R12 47k 5V Q1 MPSA05 R14 R15 1k C7 100µF 4 5 C10 10nF 2 5V 8 R VCC Q 3 CV DC 7 TR GD TH 1 6 C3 15pF C4 15pF R1 10M R3 11 RS X2 10 RTC CRSSTAL R2 9 CTC 470k 12 MR 74HC4060 Q3 7 Q4 5 Q5 4 Q6 6 Q7 14 Q8 13 Q9 15 Q11 1 Q12 2 Q13 3 T0 Fig Schmidt shaping Fig. 5 Time reference

4 Applied Mechanics and Materials Vols Range Conversion Design. This design is a multi-channel acquisition, so the analog signal inputs must be multi-channel monitoring signal, so in order to make the system work in accordance with the design requirements, multi-channel switch is essential, the design uses a CD4051 switching devices. CD4051 is equivalent to a single-pole eight-throw switch, the switch is turned on which channel the input of three address code ABC to decide. "IH" is to prohibit the end, when the "IH" =1, the respective channels are not connected. In addition, CD4051 also has another power supply terminal VEE, using as a level shifter, which makes the digital signal usually under a single set of supply conditions of work offered by CMOS can directly control the multiplexer, and this multi-way switch can transmit peak - up to 15V AC signal. For example, if the analog switch power supply VDD =+5V, VSS=0V, when VEE=-5V, as long as the digital control signal is applied to this 0~5V analog switches, you can control the amplitude range of the analog signal -5V~+5V of. The range conversion process is shown in Fig. 6. Summary The digital frequency meter is a device displaying the measured frequency, which is widely used in various fields, in many electronic devices with End complex design and a variety of functions, digital frequency meter are all used. Digital frequency Fig. 6 Range conversion flowchart meter is a typical application in measurement technique, although some digital frequency meters have complex functions, it is simple and easy to use. The design uses AT89C51 as the core, using the functions of counting and timing of microcontroller to achieve measurements within a certain time on the pulse of the gate; at last, it displays the measured frequency with LED digital display tube. System is simple, reliable, and easy to operate and has better real-time. References [1] Zhen-bao Ling, Jianfeng e, Zhengguang Sun: Design ang Research of Multifunctional Digital-Counter, Journal of Jilin University(Information Science Edition), vol. 29, no. 4, pp , [2] Haihong Xie, Ping Li, Debin Lin: Digital Cymometer Design Based on FPGA, Modern Electronic Technique, no. 18. pp , [3] Chunfang Xiao, Xu-peng Han: Design of Digital Frequency Meter Based on MCU, Electronic Design Engineering, vol. 20, no. 1, pp , [4] Jun e, Xia u: Design and realization a Cymometer Based on MCU, Journal of Qinghai University(ature Science), vol. 29, no. 2, pp , Apr [5] Liang ue: The Design of Digital Cymometer Based on Microcontroller, Science & Technology Information, no. 15, pp. 8-9, Start f<200hz? Direct measurement 200<f<2k? Select the file tenfold divide 2k<f<20k? Select 100 Divide file 20k<f<200k? Select 1000 file divider Select the appropriate gear depending on the frequency

5 Manufacturing Technology, Electronics, Computer and Information Technology Applications / Design and Implementation of Digital Frequency Meter Based on SCM /