单片机外文文献翻译--基于MSP430的FSK调制解调

中文翻译材料英文题目FSK Modulation and Demodulation With the MSP430 Microcotroller中文题目基于MSP430的FSK调制解调学院：计算机科学与技术学院专业：通信工程学生姓名：指导教师：二O一三年六月IMPORTANT NOTICETexas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinueany product or service without notice, and advise customers to obtain the latest version of relevant informationto verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability.TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extentTI deems necessary to support this warranty. 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TI does not warrant or representthat any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such semiconductor products or services might be or are used. TI’s publication of information regarding any thirdparty’s products or services does not constitute TI’s approval, warranty or endorsement thereof.Copyright 1998, Texas Instruments IncorporatedContents1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)2 Demodulation Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1 Choosing the Sampling Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2 Front End Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3 FSK Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4 Bit Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Modulation Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1 Choosing the Sampling Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2 Constructing the Look Up Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3 FSK Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 2 2 2 3 4 4 4 44 Data Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)4.1 A/D Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)4.2 D/A Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)5 Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6)6 Exercising the Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7)6.1 FSK Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7)6.2 FSK Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7)7 Example Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8)7.1 Using the MSP430C325 as Main Processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8)7.2 Example Telephone Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8)8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (10)9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (11)FSK Modulation and Demodulation With the MSP430 MicrocontrolleriFSK Modulation and Demodulation With the MSP430MicrocontrollerABSTRACTThis application report describes a software program for performing V.23 FSK modemtransceiver functions using an MSP430 microcontroller. It makes use of novel filterarchitecture to perform DSP functions on a processor with only shift and add capabilities.1 IntroductionMany measurement applications (for example, electric and gas meters) requirea way to communicate electronically with a central office so that measured datacan be reported back to the central office and new tariffs can be set in the remotesite. Telephony provides a convenient means of data communication.Frequency shift keying (FSK) and dual tone multi frequency (DTMF) are twopopular methods of representing binary data over telephone circuits. Thisapplication report describes a V.23-compliant FSK transceiver software module.Integrating the measurement and communication functions onto the same chipyields cost as well as power-saving benefits. Using the MSP430, a high MIPs ultralow power microprocessor, allows power to be drawn from the telephone line insome cases.This report describes the mathematical formulas for FSK signal transmission anddetection. A list of the software modules is included with a reference schematicfor telephone interface and low cost A/D converter. The schematic is only areference, since the precise implementation can vary from country to country.1Demodulation Theory2 Demodulation TheoryA quadrature demodulator provides the FSK demodulation. In this type ofdemodulation, the signal and its delayed version are multiplied together and then low-pass filtered. If the delay, T, is set such that Wcarrier ⋅ T = /2, then thelow-pass filter result is proportional to the frequency deviation from the carrier and therefore represents the bit value sent.If wWcarrierwhere w = 2π ⋅ f : + " Wdelta and T+ Wcarrier +p 2 ³2.1 cos[wt].cos[w(t –T)]coswT coswTsin[" Wdelta] + ) cos(2wt –wT) ³ Low Pass Filter +" sin[Wdelta]_ Choosing the Sampling RateThe sampling is chosen to be Fcarrier4 for the purpose of obtaining thedelayed sample without computational overhead. For V.23, the F carrierfrequency is 1700 Hz and therefore the sampling rate becomes 6800 Hz. Using a 32768-Hz crystal yields 6793.3 Hz, which is 0.1% out. The sampling frequency is set by the 8-bit interval timer. Because this timer is limited to 256 counts, the interrupt rated is set to twice the sampling rate and the processing is divided into two halves with signal sampling performed every other interrupt.2.2 Front End ProcessingMost A/D converters, including the successive approximation A/D converter in the MSP430C325, need a dc bias; this yields an unsigned integer sample with an offset. Before this sample can be processed further, it needs to go through an unbias filter to take out the dc bias and turn the sample into a signed integer value. This unbias filtering also gives 30 dB or so of rejection for main frequencies.2.3 FSK DemodulationThe signed integer sample and its delayed version are multiplied together; in this application, an 8×8 signed multiplication loop is used.The product, made up of two frequency elements, is low-pass filtered to remove the double frequency element. The remainder is a signed integer valuerepresenting the original bit value transmitted.The low-pass filter uses the digital wave filtering technique. This technique gives stable characteristics with very good coefficient tolerance. All multiplication is done through shifts and adds with the number of shift/add operations minimized through rounding off the coefficients. Because the filter has good coefficienttolerance, this rounding off does not affect the filter performance. The Butterworth filter used here gives approximately 40-dB attenuation in the stop band with 1-dB pass and ripple.2SLAA037Demodulation Theory2.4Bit SynchronizationThe bit values coming out from demodulation need to be determined andsynchronized to produce the incoming data bit stream. This process is alsoknown as bit slicing and clock recovery. Because the sampling rate at 6800 is notan integer multiple of the data rate (baud rate) at 1200, an additional step isneeded to consolidate between the two rates. This is done through a count-downcounter with a sequence of preload value (5,6,5). Every 17 samples, the samplingrate and the data baud rate are resynchronized. Bit synchronization or clockrecovery is done by monitoring bit value transitions. Lead or lag information isthen obtained and the count-down counter is adjusted accordingly. Because ofthe difference between the sampling clock and the data clock, the data bit is neversampled at the middle of the baud period; instead a –5% to 13% variation isintroduced. However, this should not have any adverse effect on the accuracy ofthe system, as it has been verified experimentally.3FSK Modulation and Demodulation With the MSP430 MicrocontrollerModulation Theory3 Modulation TheoryFSK modulation involves alternating the value of a delta frequency from a carrierfrequency according to the value of the bit to be represented. For V.23, a bit valueof 0 = 400 Hz and a bit value of 1 = –400 Hz.FSK signal + Amplitudecos[t| 2p(Fcarrier" Fdelta)]The sinusoidal signal is generated through a lookup table which contains cosine values from 0 to 2π. A parameter called PHASER (16 bit) represents the current angle: 0=0 degree, 8000 hex = 180 degree 10000 hex = 360 degree. With each sample, this angle is advanced by another parameter DELTA (16 bit) which determines the frequency of the signal (larger DELTA value = higher frequency). Frequency modulation is realized by changing the DELTA value according to the bit value to be transmitted at each baud period, according to the following formula:DELTA + Fdesired Fsampling65536.The advantage of this method over a digital oscillator method is that this methodpreserves the phase relationship even when the frequency is shifted from sampleto sample.3.1Choosing the Sampling RateThe 8-bit interval timer sets the sampling rate to 19200 samples/s. This rate issubdividable into the data baud rate of 1200. Also, it is sufficiently high to makethe D/A process simpler.3.2Constructing the Look Up TableTo save ROM space, only the first quadrant (0 to 127 degrees) in Q7 format iscoded. This is done by dividing the first quadrant (90 degrees) into 128 steps ofapproximately 0.7 degrees each. The remaining three quadrants can be workedout from this first quadrant table using additional computation.3.3FSK ModulationThe parameter PHASER is advanced by the amount DELTA at every interrupt.The first 9 bits of the PHASER is used to look up the cosine value. For the cosinefunction, the third and fourth quadrant are the same as the second and firstquadrant, and so only the absolute value of the first 9 bits of PHASER is used.Next, all second quadrant values are derived from the first quadrant ROM table.The 8-bit result value is stored onto P0.OUT.Every 16 interrupts, the parameter DELTA is updated with the next frequency bylooking at the next bit to be transmitted.4SLAA037Data Conversion 4 Data ConversionThis section describes the required digital-to-analog (D/A) and analog-to-digital(A/D) data conversions.4.1A/D ConversionThe most straightforward way to digitize the incoming FSK signal is to use the12-bit mode of the internal 14-bit A/D converter of the MSP430C325. However,not all of the 12 bits are needed to achieve good dynamic range for the FSKdemodulation. Simulation results indicate that an 8-bit A/D stage gives gooddynamic range up to 25 dB using internal AGC software. With an additionalexternal AGC stage, the dynamic range can be further widened. As economicalmeans of building 8-bit single slope A/D exists, this extends the application of thismodule to the rest of the MSP430 family. The application software included hereuses a single slope A/D (universal timer with external comparator) for thedemodulator. This makes the software universally applicable for the whole family.4.2D/A ConversionA 6-bit external R–2R ladder is used to construct the D/A converter. Because thecarrier frequency of 19200 Hz is nine times the highest frequency of the FSK of2100 Hz, the post filtering stage should be relatively simple. In the applicationcircuit, a single capacitor forms a single pole low pass filter but more poles canbe realized using additional passive networks.5FSK Modulation and Demodulation With the MSP430 MicrocontrollerPower Consumption5 Power ConsumptionThe FSK concept is designed with low power in mind. The FSK demodulatortakes less than 2 MIPs. With a low power op-amp as a front-end, total powerconsumption of less that 1.5 mA should be achievable. Thus, it is possible thatthe power can be derived entirely from the telephone line. A schematic is includedfor a suggested telephone line interface. The precise configuration may vary fromcountry to country.SLAA0376Exercising the Software 6 Exercising the SoftwareThis section describes operation of the software.6.1FSK ReceiverThe FSK signal is derived from the telecom interface circuit. This signal shouldhave a dc bias of 1.2 V and a peak-to-peak level of 400 mV. The software decodesthis FSK signal and produces three outputs which lets the user monitor thedemodulated data.TP.3. This is the clock signal recovered from the input FSK.TP.5. This is the data recovered from the input FSK; data is latched out everyrising edge of TP.3.P0.2–P0.7. These six bits output the low pass filtered result. With an externalR–2R ladder this becomes very useful in monitoring the analogue FSKdemodulator output level. It is hard limited to 8 bits with the MSB 6 bits loadedto port P06.2FSK TransmitterThe transmitter software outputs an FSK signal according to the BIT MAP datadefined in TX_DATA_TABLE. The bitmap pattern starts with a preamble followedby a long MARK period. Then the actual data is transmitted. This table uses a zeroword as an end marker, and the software restarts the whole data sequence uponreaching a zero value in the bit map data.7FSK Modulation and Demodulation With the MSP430 MicrocontrollerExample Circuits7 Example CircuitsThis section shows and describes example circuits.7.1Using the MSP430C325 as Main ProcessorFigure 1 shows an example circuit using the MSP430C325 as the mainprocessor. The circuit is tested with 400 mV peak-to-peak FSK input. To obtainthe same results, Rx needs to be biased at 1.2 V with a 400 mV peak-to-peak FSKsignal superimposed.VSSR1 R2PO.2PO.3PO.4PO.5PO.6PO.7MSP430E325TP.5TP.1 TP.4 CIN TP.3RX_CLKLine InterfaceRXTXHook 14066AC13VCC1N414833 kΩVoltage RampPNPSample_HoldNPN 1 nF 6_ 5+B 2RX_DATA7Figure 1. Main Processor and A/D Converter7.2Example Telephone InterfaceFigure 2 shows an example telephone interface, and Table 1 lists FSK transceiverperformance data.8SLAA037Example Circuits 20 kΩ1 ∝Φ+ 1 kΩ1 kΩVREF(1.5 V)TLC22796_5+33 kΩ20 kΩ20 kΩ20 kΩ10 kΩ9_10+Telephone LineAB33 nF500 &6–8 V ZenersTuning ForMinimum Side Tone6–8 V ZenersTX7DC TelephoneIsolationTransformer8150 kΩ400 mV pk–pkRX 1 ∝Φ+–+131233 kΩ33 kΩ680Hook150 kΩ14This is a reference circuit only and may not be applicable under some circumstances.Figure 2. Telephone InterfaceTable 1. FSK Transceiver PerformanceRAM (BYTES)FSK Receiver FSK Transmitter1812ROM (BYTES)512400MIPS (APPROX.)21.4FSK Modulation and Demodulation With the MSP430 Microcontroller9Summary8 SummaryFSK transceivers are normally realized by either analog means or by the use ofDSPs with hardware MAC units. Using an MSP430 RISC processor without ahardware MAC to achieve the transceiver function is a very unusual approach.The ability to create filters using digital wave filtering techniques, together with theorthogonal instruction set and the 16 bit architecture of the MSP430, makes thecode very ROM and MIPs efficient. Moreover, the ultra low power capability of theMSP430 means that power can readily be derived from the phone line. This leadsto component-efficient designs. The author has conducted other tests toconclude that, with some enhancements, the FSK receiver can work with an 8-bitA/D converter with enough sensitivity. Therefore the FSK transceiver can beimplemented economically across the whole MSP430 family.SLAA03710References9 References1. Texas Instruments: MSP430 Family, Architecture User’s Guide and ModuleLibrary.2. Texas Instruments Digital Signal Processing Application with the TMS320Family Volume 2.3. Gaszi, L: Explicit Formulas for Lattice Wave Digital Filters; IEEE Trans. OnCircuits and Systems VOL. CAS-32, NO. 1, January 198511FSK Modulation and Demodulation With the MSP430 Microcontroller基于MSP430的FSK调制解调——应用报告声明德州仪器（TI）及其附属公司（TI）保留改进产品或停止任何服务的权力，并且不再另行通知，建议客户获核实最新版本或相关信息，在下订单前，该信息是当前最有效和完整的。