CSEP567 Lab 6: “Pulse Width Modulation”

Objectives

The goal of this lab is to generate various colors using a tri-color LED. To select the color to be output we will create a virtual knob by measuring accelerometer readings through pulse width measurements. In addition, you will also use pulse width modulation to control the brightness of the LEDs. In this lab you will learn the following:

• how to read an accelerometer reading via pulse width measurement ;
• how to use the input capture on the 16-bit timer on the ATmega16 to do so; and
• how to adjust the intensity of a light using pulse width modulation.

Important Warnings

Do NOT do either of these two things:

1. The ADXL202EB (the accelerometer evaluation board used in the lab) is not reverse polarity protected. Do not reverse the +5V and ground terminals as it will damage the ADXL202EB and make the part unusable.
2. Do not drop the ADXL202EB on a hard surface as it may generate several thousand g's of acceleration, enough to damage the accelerometer.

Hints

• Take this lab step by step and incrementally test your code to make sure that it is working as you expected. Make sure to test pieces of code one at a time and convince yourself that they are working before moving on to another piece.
  
• The lab is broken up into multiple parts to make the lab simpler with smaller/simpler tasks that will build to the final result. It is a good idea to save a copy of your code after each step. If for some reason you are unable to complete the lab partial credit will be givin for each completed step.
  
• Utilize theLCD and the 7-segment LED displays for debugging. WARNING: If you update the 7-segment LED displays each time through your main loop you might not be able to read the numbers because it will be changing the number on the order of milliseconds.
  
• With a small enough period, a human eye should not be able to perceive any flickering of the LED when the button is not pressed. If you are seeing flickering it is most likely timing related, such as an interupt being too long. You should try to eliminate any flicker from your LED's, however, you should focus on completing the lab first instead of spending huge amounts of time trying to troubleshoot. 
  
• Focus on making your system have a reasonable interface. Do not get stuck trying to get timing calculations to work out perfectly. The accelerometer is inherently noisy. The important thing is to create an interface that “feels” right to a human. People will not notice timing errors on the order of microseconds (maybe not even a few milliseconds depending on where it is in the code).
  
• We are using 2g accelerometers – that means they detect accelerations up to two times the force of gravity. The parts should not be subjected to too much more than this – being dropped or banged on a hard surface. Your calculations can assume that you are only using gravity to determine how far a user has turned the accelerometer to the left or right. By using this assumption your calculations will be incorrect because they will not take into account the force you yourself apply in starting and/or stopping the accelerometer. Treat this(the starting/stopping acceleration) as noise as it is difficult to account for this systematically.

Suggested Reading

• The following sections of the ATmega16 datasheet might be helpful for this lab:
  • Interrupts
  • 8-bit Timer 0
  • 8-bit Timer 2
  • 16-bit Timer 1
• Accelerometer(ADXL202) Datasheet
• Accelerometer Application Note on using the Duty Cycle Output
• Tri-Color LED Datasheet  Note: We will be using Common Anode LEDs.

Resources

Brief introduction to AVR Programming
avr-gcc manual
Application notes section for the AVR 8-bit RISC family
Accelerometer(ADXL202)Datasheet
AccelerometerApplication Note on using the Duty Cycle Output
Tri-Color LED Datasheet Note: We will be using Common Anode LEDs.
Color Applets

Suggested Steps

PART 1:

1. Obtain an accelerometer and cable.Wire your accelerometer module into your breadboard with the cable. The cable for your accelerometer module uses a black wire for ground, red for +5 volts, yellow for the Y-axis output, white wire for the X-axis output, and blue wire for the button (the other end of the button is connected to ground). Attach the Y-axis output to ICP1 (pin 20) and the blue wire to the INT1 pin to detect button presses. Remember to add a pull-up resistor to the button circuit. Plug the X-axis output into an unused row on your breadboard as we will use it later in the lab.
2. Refer to page 9 of the accelerometer datasheet to determine what resistor to add to the accelerometer board at R1. R1 should be chosen such that the period of the accelerometer’s PWM period is approximately 4ms. Trim the leads to fit.
3. Review important warnings (YES, AGAIN) and apply power to the circuit. Observe the y-axis output with an oscilloscope. Measure the period and min-max duty cycle.

   Complete Worksheet.

PART 2: (Test your code from Part 1 from the prelab)

1. Implement a program that uses timer 1 input capture mode, (accelerometer y-axis output to ICP1 pin 20), to determine the positive duty cycle of the y-axis accelerometer. To do this, time the length of the positive pulse of your accelerometer (rising edge to falling edge) and the length of the period (rising edge to next rising edge). Use the 7-segment LED to display a "1" when the accelerometer has a duty cycle less than 50% and a "2" when the accelerometer has a duty cycle greater than or equal to 50%.
   NOTES:
• You should choose as small a prescaler factor as possible for Timer 1 to increase the accuracy.
• Assume a period of 4 ms. for the Accelerometer outputs.
• Use TCNT1 and ICR1 to access 16-bit values instead of using the 8-bit registers (i.e. ICRL & ICRH and TCNT1L & TCNT1H)
• Remember to set TCNT1=0 when you want the counter restart at 0. The input capture interrupt does not reset TCNT1 to zero automatically

PART 3: (Test your code from Part 2 from the prelab)

1. New program: Define the x-axis of the accelerometer as an input on INT0 (pin 16).
2. Use INT0 and Timer2 to create a manual interupt capture routine to determine the positive duty cycle of the x-axis.
3. Implement a program that measures the x-axis and drives the 7-segment LED as in part 2. 

PART 4: (Integrate Parts 2 & 3 and HSV)

GOAL: To generate various colors on the tri-color LED using the accelerometer to select color and the potentiometer to adjust brightness. A user should be able to press the button and move the accelerometer until they find a color they want. On button release the H & S value should lock in. A user should be able to update the brightness of the color they choose by adjusting the potentiometer at any time.

1. Use the provided HSV to RGB code to control the tri-color LED. Code
   H->x-axis
   S->y-axis
   V->ADC value of potentiometer (this controls brightness)
2. To eliminate gitters make sure to average your values (H, S, & V).
3. The color should only change while the user holds the button on the accelerometer down. Brightness should update anytime the value of the potentiometer is changed.
4. Suggested timer uses:

• Timer0 -> used to generate the 3 PWM’s needed for the tri-color LED
• Timer1 -> input capture for the y-axis (possible button polling)
• Timer2 -> used with INT0 to perform input capture for the x-axis

Deliverables

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

• Both partners' full names.
• The lab number and the part of the lab (e.g. “Lab 6”).

1. Demonstrate part 4 to a TA. You can either do this during this lab, or during the first 1/2 hour of the next lab.
   • You will not receive full points if there is noticible flicker on any of your LED's when the button on the acclerometer is not pressed.
   • The light levels of the LED's should not change until the button is pressed or your potentiometer is adjusted (which should only adjust the brightness).
   • It should be possible to display all major colors on your Tri-Color LED (The more distinct colors the better!).
2. Turn in hardcopy of your commented C code.
   • Make sure that the code in your interrupt handler is as minimal as possible. It is fine to update state or do a little bit of work in the interrupt handler. We will grade on how you designed your program to minimize code in the interrupt handler.