{"id":502,"date":"2018-10-17T10:27:05","date_gmt":"2018-10-17T10:27:05","guid":{"rendered":"http:\/\/blogs.plymouth.ac.uk\/embedded-systems\/?page_id=502"},"modified":"2019-09-20T15:18:54","modified_gmt":"2019-09-20T15:18:54","slug":"topic-5-analogue-output","status":"publish","type":"page","link":"https:\/\/blogs.plymouth.ac.uk\/embedded-systems\/microcontrollers\/mbed-os-2\/courses\/embedded-systems-in-context-level-4\/topic-5-analogue-output\/","title":{"rendered":"Topic 5 – Analogue Output (introduction)"},"content":{"rendered":"

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

\n

This is section,we look at ways to output analogue signals.<\/a> For this we use two techniques: Pulse Width Modulation (PWM)<\/a> and a Digital to Analogue Converter (DAC)<\/a>. The microcontroller we are using does in fact have an on-chip DAC, but as no all devices do, we have included an example of interfacing to an external device. We achieve this using an industry de facto standard known as the Serial Peripheral Interface (SPI).<\/p>\n

Intended Learning Outcomes<\/h1>\n
    \n
  1. Define the meaning of PWM and calculate the RMS voltage of a PWM signal<\/li>\n
  2. Use PWM to create a known average voltage<\/li>\n
  3. Program a PWM peripheral<\/li>\n
  4. Program the internal DAC with the DigitalOut class<\/li>\n
  5. Use the SPI interface to interface to an external DAC<\/li>\n<\/ol>\n
    \n

    Activity 5.1 Pulse Width Modulation (PWM)<\/h1>\n

    Pulse Width Modulation<\/a> (or PWM) is a simple technique used to generate an alternating voltage signal with an known \u201caverage DC voltage\u201d. The technique is commonly used in control systems, including the control of DC motors.<\/p>\n

    TASK 5.1.1<\/h2>\n\n\n\n\n
    Read the glossary entry on PWM<\/a><\/td>\n<\/tr>\n
    \u00a0Attempt the short quiz below.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

     <\/p>\n

    Q1.For a PWM system, using a pulse of 1V, \"T=100 \mu s\" and \"T_{mark} = 10 \mu s\", which of the following is true?
    \n(hover your mouse over the correct answer).
    \nA. The duty is 0.1%<\/a>
    \n    B. The duty is 10%<\/a>
    \n    C. The mean voltage is 0.1\u03bcV<\/a>
    \n    D. The mean voltage is 10V<\/a><\/p>\n

    Q2. For a PWM system, using a pulse of 10V, \"T=100 \ ms\" and \"T_{mark}=1 \ ms\", what is the mean voltage?<\/p>\n

    A. 0.1V%<\/a>
    \n    B. 1V<\/a>
    \n    C. 10V<\/a>
    \n    D. 100V<\/a><\/p>\n

    Q3. For a PWM system driving a DC motor, using a pulse of 5V, \"T=1 \ ms\" and \"T_{space}=500 \mu S\", which of the following is true?<\/p>\n

    A. The choice of T means motor would be louder than applying a constant voltage<\/a>
    \n    B. This makes no sense as Tmark<\/sub> > T<\/a>
    \n    C. The output voltage would be 3.3V<\/a>
    \n    D. The duty is 20%<\/a><\/p>\n

    Activity 5.2 Pulse Width Modulation (PWM) with Timers<\/h1>\n

    In this task we will implement PWM using a a simple timer.<\/p>\n\n\n\n\n\n
    Build and run the code shown below (also on mbed)<\/td>\n<\/tr>\n
    What are the ON and OFF times?<\/td>\n<\/tr>\n
    Try different values of R<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    #include \"mbed.h\"\r\n\r\n\/\/Time period\r\n#define T 0.001\r\n\r\n\/\/Mark-Space Ratio\r\n#define R 0.1\r\n\r\n\/\/Mark and Space times\r\n#define Tmark (R*T)\r\n#define Tspace ((1.0-R)*T)\r\n\r\nDigitalOut onboardLed(LED1);\r\nDigitalOut redLED(D7);\r\n\r\nint main() {\r\n   printf(\"\\nWelcome to ELEC143\\n\");\r\n   \r\n   while (1) {\r\n      redLED = 0; \/\/Space\r\n      wait(Tspace);\r\n      redLED = 1;\t\/\/Mark\r\n      wait(Tmark);\r\n   }\r\n}<\/pre>\n
    Task 5.2.2<\/h2>\n
    Perform the following:<\/p>\n\n\n\n\n\n
    Now try and modify task 5.2.1 to also independently<\/b> set the brightness of the green LED. If you spend more that 10 minutes on this task<\/em>, maybe stop. The point of this exercise is to illustrate the difficulty (there is an easier way)!<\/td>\n<\/tr>\n
    If you have succeeded, consider how you might now also control the brightness of the yellow LED?<\/td>\n<\/tr>\n
    Describe what problems you encounter<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    There are many ways to try and solve this problem. I have gone for a solution<\/a> that uses timer interrupts (although even this is not perfect).<\/p>\n
    What you might find is that your solution does not \u201cscale\u201d beyond one or two LEDs. One of the fundamental problems here is that the wait statement is \u201cblocking\u201d (you cannot do anything else unless you use an interrupt).<\/p>\n
    There are a number of elegant solutions. Timers are one possibility (as I have used), but you might want to reserve those timers for other tasks. I\u2019ve kept the interrupt routines as short as possible to avoid latency issues (in the event two overlap in time such that one has to wait for the other to finish).\u00a0<\/span><\/p>\n
    Luckily, PWM is a common requirement and this problem was recognized a long time ago, and this microcontroller comes with PWM controllers built in! These controllers have their own timers and logic to control the output, so all run independently in parallel.\u00a0<\/span><\/p>\n
    Remember:<\/b> a single core microcontroller can only really execute one instruction at a time. Any attempt at multitasking is always an illusion and often comes with compromises.\u00a0<\/span><\/p>\n
    Look at the code below to see how simple it is to use a PWM in Mbed-os on this device. Note for the F429 board, I’ve moved the RED LED to pin D6. You should modify your hardware accordingly.<\/p>\n
    #include \"mbed.h\"\r\n\r\nDigitalOut OnBoardLed(LED1);\r\n\r\n#ifdef TARGET_NUCLEO_F429ZI\r\nPwmOut pwmRed(D6);  \/\/D7 is not a PWM output on the F429, so use D6 instead\r\n#else\r\nPwmOut pwmRed(D7);\r\n#endif\r\n\r\nint T = 10;\r\nint Tmark = 5;\r\n\r\nint main() {\r\n    \r\n    pwmRed.period_us(T);\r\n    pwmRed.pulsewidth_us(Tmark);\r\n    \r\n    while(1) {\r\n        \/\/Heartbeat\r\n        wait(1);\r\n        OnBoardLed = !OnBoardLed;\r\n    }\r\n}<\/pre>\n
    The following should be noted:<\/p>\n