{"id":821,"date":"2019-10-10T08:58:33","date_gmt":"2019-10-10T08:58:33","guid":{"rendered":"http:\/\/blogs.plymouth.ac.uk\/embedded-systems\/?page_id=821"},"modified":"2019-10-24T11:07:54","modified_gmt":"2019-10-24T11:07:54","slug":"analogue-interfacing-real-time-data-conversion","status":"publish","type":"page","link":"https:\/\/blogs.plymouth.ac.uk\/embedded-systems\/microcontrollers\/mbed-os-2\/courses\/level-5-embedded-and-real-time-systems\/analogue-interfacing-real-time-data-conversion\/","title":{"rendered":"Analogue Interfacing – Real Time Data Conversion"},"content":{"rendered":"

Table of Contents<\/a><\/p>\n

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In this section we look at interfacing with the analogue world using mbed. You will use a simple potentiometer<\/a> to generate analogue voltages, and measure that voltage using an Analogue to Digital Converter (ADC)<\/a> to perform sampling. Analogue signals suffer problems with noise and uncertainty. Analogue filters are used to prevent a phenomena known as aliasing when sampling a signal. For simple threshold detection, hysteresis<\/a> is introduced to manage signal noise and avoid output jitter.<\/p>\n

\"Image<\/p>\n

We then look at digital to analogue conversion, using both pulse-width modulation (crude, but useful for motor speed control) and a Digital-to-Analogue Converter (DAC<\/a>) for more precise applications.<\/p>\n

The mechanisms for conversion in mbed are very simple.<\/p>\n

What is more important are the techniques for determining an accurate sampling rate and reducing aliasing noise.<\/p><\/blockquote>\n

For accurate sampling, we will use the Ticker<\/code> component to generate interrupts driven by a hardware timer.<\/p>\n

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For a simple threshold detection task such as this, we might can use \u201chysteresis<\/a>\u201d to address the problem of noise.<\/p>\n

Hysteresis<\/h2>\n

This is a technique for reducing (and sometimes effectively eliminating) the effects of noise when detecting if a signal crosses a threshold. Consider the figure below. This shows an analogue signal \"V_{in}(t)\" varying with respect to time \"t\".<\/p>\n

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Consider the example of the blue waveform in the figure. The signal is below the lower threshold at \"t=0\", so the output is GREEN. Hysteresis<\/a> works as follows:<\/p>\n