{"id":371,"date":"2018-10-02T07:54:09","date_gmt":"2018-10-02T07:54:09","guid":{"rendered":"http:\/\/blogs.plymouth.ac.uk\/embedded-systems\/?page_id=371"},"modified":"2019-09-18T15:30:55","modified_gmt":"2019-09-18T15:30:55","slug":"topic-4-analogue-input","status":"publish","type":"page","link":"https:\/\/blogs.plymouth.ac.uk\/embedded-systems\/microcontrollers\/mbed-os-2\/courses\/embedded-systems-in-context-level-4\/topic-4-analogue-input\/","title":{"rendered":"Topic 4 \u2013 Analogue Input"},"content":{"rendered":"

Back to ToC<\/a><\/p>\n

In this section we look at interfacing with the analogue world. You will use a simple potentiometer<\/b><\/a> to generate analogue voltages, and measure that voltage using an Analogue to Digital Converter (ADC)<\/a><\/strong>.\u00a0We also look at different ways to represent an analogue voltage as a digital value.\u00a0Hysteresis<\/b><\/a> is used to manage signal noise and avoid output jitter.\u00a0You will also discover problems with noise, and use \u2018hysteresis\u2019 to avoid instability due to noise.<\/p>\n

Intended Learning Outcomes<\/b><\/h1>\n
    \n
  1. Use a potentiometer<\/a> to generate and control an analogue signal<\/a><\/li>\n
  2. Convert an analogue signal to an integer digital representation using an analogue to digital converter<\/a><\/li>\n
  3. Scale an integer value to a fractional value<\/li>\n
  4. Use hysteresis<\/a> thresholds to avoid problems with noise<\/li>\n
  5. Use conditional statements<\/a> and operators to make decisions in software<\/li>\n
  6. Break a program into logical and reusable functions<\/li>\n
  7. Use looping commands to perform iteration to average a measurement<\/li>\n
  8. Use a hardware interrupt timer for accurate sampling.<\/li>\n<\/ol>\n

     <\/p>\n

    \n

    Activity 4.1 Potentiometer<\/h1>\n

    In this section, we are going to use a “potentiometer<\/a>” to create an analogue signal<\/a>.\u00a0Your potentiometer<\/a> should look something like the figure below.<\/p>\n

    \"\"
    Photo of the Potentiometer used in this course<\/figcaption><\/figure>\n

    Below is a diagram for the prototype board. We have simply added a potentiometer to the circuit, with the middle terminal connected to pin A0.<\/p>\n

    \"\"
    The prototyping board layout is shown above. Note the power supply rails have been linked to extended the power rails.<\/figcaption><\/figure>\n

    The schematic for the potentiometer is shown below.<\/p>\n

    \"\"
    Potentiometer Circuit schematic, providing a variable voltage input to the analogue input A0<\/figcaption><\/figure>\n

     <\/p>\n

    \"\"
    (unofficial) Symbol for the potentiometer for prototyping diagrams<\/figcaption><\/figure>\n
    <\/div>\n

    Task 4.1.1<\/h2>\n

    Perform the following steps:<\/p>\n\n\n\n\n
    Wire the circuit shown. Instead of connecting the wire to A0, connect it to a DVM on the voltage range.<\/td>\n<\/tr>\n
    By rotating the potentiometer (POT), what range of voltages do you observe?<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

     <\/p>\n\n\n\n\n\n\n\n\n\n\n
    Now connect the center terminal wire to A0 (as shown in the figure above)<\/td>\n<\/tr>\n
    Create new project, paste in the code below, compile and and run.<\/td>\n<\/tr>\n
    Run the terminal (PuTTY) to see the output. If nothing appears, you may need to change the COM port setting (use Windows Device Manager to check)<\/td>\n<\/tr>\n
    Set the POT to the two extremes and observe the values<\/td>\n<\/tr>\n
    Set the POT such that you get close to 0.5.<\/td>\n<\/tr>\n
    Are you able to get precisely 0.5, and if not, why<\/b>?<\/td>\n<\/tr>\n
    Using an if-else<\/a> statement, write some additional code to switch the\u00a0<\/span>Green LED<\/a> ON when the analogue input is between 0.0 and 0.5, the Red LED ON when greater than 0.5 and 1.0. A solution is available here<\/a>. See the glossary entry about if-else if-else<\/a> if you are unsure.<\/td>\n<\/tr>\n
    Set the POT to halfway, and try and make the LED\u2019s switch on and off erratically.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    Sometimes we might write the specification as follows:<\/p>\n

      <\/span>   <\/span>\"\[<\/p>\n

    #include \"mbed.h\"\r\n\r\n\/\/Global objects\r\nBusOut binaryOutput(D5, D6, D7);    \r\nDigitalIn SW1(D3);\r\nDigitalIn SW2(D4);\r\n\r\n\/\/Analog input\r\nAnalogIn AIN(A0);\r\n\r\n\/\/Global variable\r\nfloat fVin = 0.0;\r\n\r\n\/\/Main function\r\nint main() {\r\n    \r\n    while(1) {\r\n        \r\n        \/\/Read ADC\r\n        fVin = AIN;\r\n        \r\n        \/\/Write to terminal\r\n        printf(\"Analog input = %5.3f\\n\", fVin); \r\n        \r\n        \/\/Wait\r\n        wait(0.5);\r\n        \r\n    } \/\/end while(1)\r\n} \/\/end main<\/pre>\n
    AnalogIn<\/strong><\/h3>\n
    In this example we introduce the Mbed-os type AnalogIn<\/span>. This enables use to access an on-chip analogue to digital converter<\/a> that converts an analogue signal<\/a> to a digital representation. We create a variable of type\u00a0AnalogIn<\/span> much like we did with DigitalIn<\/span>\u00a0:<\/p>\n
    AnalogIn AIN(A0);<\/pre>\n
    Like DigitalIn<\/span>\u00a0, the single parameter is the pin name A0<\/span>. We read the analogue input value as follows:<\/p>\n
    fVin = AIN;<\/pre>\n
    Note the data type of fVin<\/span>\u00a0 is float<\/span>. This type of data can hold fractional numbers, as opposed the int<\/span>\u00a0 and unsigned int<\/span>\u00a0 data types that only hold whole numbers. Using this method, the value we read is conveniently scaled between 0.0 (0V) and 1.0 (3.3V).<\/p>\n
    Observations<\/h3>\n
    No signals are ever perfectly constant. The observed value we see changes due to electrical noise.\u00a0<\/span>Noise is always<\/i> present, so as the value gets close to 0.5, there will always be some point where the output \u201cjitters\u201d between RED to GREEN. The output is unstable at this point.<\/p>\n
    Consider the case where the output is not an LED, but something more critical:<\/p>\n