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Interfacing PIC Microcontrollers 26. 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 Amplifier Design In order to design the interface between a sensor and the MCU we need to specify the performance of a linear amplifier which will translate the output of the sensor into a suitable input for the MCU which is in the right range for the amplifier to handle and allows a convenient conversion factor to be used in the ADC. For example our standard temperature sensor with built-in signal conditioning produces an output of 10 mV per degree C with an output range of 55 to 150 C. Let us assume it is to be interfaced to a PIC MCU to measure between 10 and 35 C. Gain At 10 C the input will be 10 X 10 100 mV At 35 C it will be 35 X 10 350 mV The gain is calculated as the required change in the output divided by the range of the input. Now if we are using a single supply op-amp package such as the LM324 the output range is strictly limited. The output only goes down to about 50 mV and up to about 3.5 V. So if we assume an output swing of 2.5 V is available the gain required 2.5 0.35-0.1 10. Offset As in this case sensors often have a positive offset so the output range has to be shifted down. With a gain of 10 the output at the lowest temperature will be 100 mV X 10 1.00 V and at the highest 350 mV X 10 3.50 V. This can be shifted down by 1.00 V so that the output range will be 0-2.50 V. This also allows us to use a reference voltage of 2.56 V just above the required maximum which gives a convenient 8-bit conversion factor 10 mV bit . Including 0 V in the output means that we will lose the lowest degree or two from the range. If this is not acceptable the offset can be adjusted to suit so that the output ranges from 0.5 to 3.0 V. In this case the negative reference voltage for the ADC should be modified to 0.5 V and the positive reference to 3.06 V. If necessary corrections can also be made in software. Frequency Response Since we are measuring direct voltages it is sensible to restrict the frequency response of the amplifier to
