TAILIEUCHUNG - Interfacing PIC Microcontrollers 25

Interfacing PIC Microcontrollers 25. Explore in detail microcontroller interfacing techniques using the popular PIC 16F877 Work through step-by-step examples interactively using circuit simulation software, supplied as assembly source code Gain the knowledge of a wide range of peripheral devices such as keyboards, displays, sensors and drives and serial communication with other processors, memory and more Use interactive simulation software to design and test circuits. | Interfacing PIC Microcontrollers digital conversion. Suitable signal conditioning may be needed using amplifiers filters and so on to produce a clean signal controlling noise drift interference and so on with the required output range. Sensors have certain characteristics which should be specified in the data sheet Sensitivity Offset Range Linearity Error Accuracy Resolution Stability Reference level Transfer function Interdependence The meaning of some of these is illustrated in Figure . SENSITIVITY The ideal sensor characteristic is shown in the characteristic y m1x. The sensor has a large change in its output for a small change in its input that is it has high Figure Sensor characteristics 226 Sensor Interfacing sensitivity. The output could be fed directly into the analogue input of the MCU. The line also goes through the origin meaning no offset adjustment is required -a linear pot would give this result. If the sensor has low sensitivity y m2x an amplifier may be needed to bring the output up to the required level. OFFSET Unfortunately many sensors have considerable offset in their output. This means that over range for which they are useful the lowest output has a large positive constant added y m3x c . This has to be subtracted in the amplifier interface to bring the output back into the required range where maximum resolution can be obtained. The same can be achieved in software but this is likely to result in a loss of resolution. Temperature sensors tend to behave in this way as their characteristic often has its origin at absolute zero 273 C . The sensor may have offset and negative sensitivity such as the silicon diode temperature characteristic y m4x c2 . In this case an inverting amplifier with offset is needed. LINEARITY The ideal characteristic is a perfect straight line so that the output is exactly proportional to the input. This linearity then has to be maintained through the signal conditioning and conversion processes. Metal .

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