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Understanding Smart Sensors - Nomads.usp

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106 <strong>Understanding</strong> <strong>Smart</strong> <strong>Sensors</strong>Table 5.2Differences Between DSP and MCU ArchitecturesCharacteristic DSP MCUEase of programmingBoolean logicReal-time operating efficiencyCode maintainabilityPeripherals availabilityBetterBetterBetterBetterBetterHowever, code portability is minimal in today’s level of DSPs with the strategyclosely linked to the I/O and the architecture. Also, generating code for a DSP,in general, is more difficult than generating code for an MCU. However, themultiply-accumulate unit programmed in C requires less effort. As moresophisticated tools are available to deal with DSPs at the behavioral level, thecurrent difficulties in programming fast Fourier transforms (FFTs) and developingfilters for DSPs will be simplified.Figure 5.6 shows a 24-bit DSP and its architecture. The device has programmemory, two data memories, and the same kind of instruction set that iscommon throughout the Motorola MCU family. Using similar instruction setsmakes it easier to write code and minimizes the discontinuity for programmersfamiliar with the MCU family. For example, the multiply, add, move, branch,and bit test are the same. However, the advance features of the chip require newinstructions.A DSP has been used to develop an intelligent sensor for checking thepitch of tapped holes in motor blocks. The sensor replaced a mainframe computerthat was checking one hole per minute. By providing DSP power in eachsensor, the inspection system increased the rate to 100 holes per minute [8].The sensor was developed using a DSP design kit that is the size of a creditcard. Voltage monitoring, battery backup circuitry, and a watchdog timer arefeatures of the design kit and the DSP-based sensor.5.4.1 Algorithms Versus Lookup TablesMCUs use lookup tables to store values that are accessed when the program isrunning. Algorithms are used to correct for variations from expected results andto implement a control strategy. The speed of accessing the information from a

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