CSE466 Lab 2: Timer Interrupts and Pulse-Width-Modulaton

Objectives

The goal of this lab is to introduce various features and modules of the MK20DX256 while introducing how to program the microcontroller using a timer interrupt. In this lab you will learn the following:

This lab assignment has 4 required sections. The deliverables section below outlines exactly what should be turned in.

Helpful Hints

Suggested Reading

Suggested Steps

Section A: the IntervalTimer

IntervalTimer uses interrupts to call a function at a precise timing interval. Here is a reference for IntervalTimer, and sample code for it's use.

Copy the sample code into a new Arduino Sketch, calling it MyIntervalTimer.

Compile and run the code, and open the Arduino serial debug window. Study the working code, paying attention to the comments, and then answer the following questions.

Question 1: What starts the timer running?

Question 2: What function executes when the timer interrupt occurs?

Question 3: What section of the code is interrupted by the timer interrupt?

Question 4: What is the purpose of the following functions:

noInterrupts();
interrupts();

Save your working sketch for future reference and use.

Section B: Oscilloscope Tutorial

  1. Go through the entire Oscilloscope Tutorial (consult the Oscilloscope Manual as needed).
  1. Here is a reference on bypass capacitors, which are used to reduce noise in your circuit: http://www.seattlerobotics.org/encoder/jun97/basics.html.

    Question 5: What is the purpose of the bypass capacitors? Why might they be useful for this lab?

Section C: Tri-color LED and Blinking

  1. Add the RGB LED to your breadboard.

    WARNING: You must use a current-limiting resostor in series with the cathodes of the LED. Failure to do so will result in damage to the LED. In addition, be very careful when wiring that you do not creat any short-circuits between any of the power pins and ground. You could damage the Teensy board.

    Here is the pinout:
    Tricolor LED
    Pinouts:
    Pin 1. Blue
    Pin 2. Green
    Pin 3. Common Anode
    Pin 4. Red


  2. You should test your LED before connecting it to the Teensy board by briefly grounding the current-limiting resistors to ground, which should light the LED. Ask if you have a problem.
    After testing, connect the LED Common Anode to the Teensy corner pin marked Vin (3.7 to 5.5 volts), the red segment to pin 20 via a 560Ω current-limiting resistor, the green segment to pin 21 via a 330Ω current-limiting resistor, and the blue LED to pin 22 via a 300Ω current-limiting resistor. It is okay to substitute the resistors for nearby values if the exact ones are not available, but err on the side of less current through the LED instead of more. Resistors are in the rightmost black cabinet on the back wall of the lab, next to the door of 003D.


    Color Codes

  1. Next, Write a new version of your IntervalTimer sketch from Section A called tricolorTimer, and modify it to blink each color for one second in succession: Red, Green, Blue. You will need to keep track of three states. Your variable blinkCount could be used as a modulo-3 counter. The Arduino serial debug window can be used to check your variable .Note that since the LED is common-anode, OFF is a 1 and ON is a 0 on the respective port pins.

    Demonstrate your tricolorTimer sketch to your TA.

Section D: Pulse-Width Modulation

  1. For Pulse Width Modulation we will use the analogWrite() function in Arduino (see references in Suggested Reading). Read that material to start.
  2. Construct a new version of your tricolorTimer sketch called PWMtimer which fires the timer every 10 milliseconds. Your code should change a variable for each LED in succession, so that RED fades up (255 to 0) and then down (0 to 255), followed by Green fading up and down, and then by Blue fading up and down, and the cycle repeats.

  3. Finally, see if you can incorporate your potentiometer and ADC code from lab one to change the fade rate. You may need to back up to your blink sketch in Lab 1 which uses the delay function to set the timing of your analogWrite() function calls. Call the new sketch ADCfade.

Question 6: Is the LED affected by the period of the PWM output, or by the duty cycle (on time), or both? Can you run it too fast or too slowly?

Demonstrate your PWMtimer and ADCfade sketches to your TA.

Deliverables

E-mail a single .zip file containing all required documents to cse466-tas@cs.washington.edu

For all files turned in, the comments at the top of the file should contain:

Answers to the SIX Lab Questions.
Commented Sketches for Sections C and D (tricolorTimer, PWMtimer, and ADCfade).