CSE466 Lab 3: “Pulse Width Modulation and Electric Field Sensing”

Objectives

The goal of this lab is to control the color of a tri-color LED pulse-width modulation.  You will control the color of the LED with an accelerometer and with an electric field sensor that you will construct.

You will learn the following concepts:

• how to use PWM to control the brightness of an LED
  
• how to use input capture to determine the duty cycle of a PWM input
• how to implement an electric field sensor

Hints

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
• Tri-Color LED Datasheet  Note: We will be using Common Anode LEDs.
• Accelerometer Datasheet
  
• Accelerometer application note on duty cycle output
  

Resources

• Brief introduction to AVR Programming
  
• avr-gcc manual – located at: C:\WinAVR\doc\avr-libc\avr-libc-user-manual-1.4.4.pdf
  
• Application notes section for the AVR 8-bit RISC family
  
• Tri-Color LED Datasheet Note: We will be using Common Anode LEDs.
  
• Accelerometer Datasheet
  
• Accelerometer application note on duty cycle output
  
• TLC084 Op-amp Datasheet
  
• Schematic for e-field sensor
  

Suggested Steps

PART 1:

1. Read all of the directions for Part 1 before doing ANYTHING. Often the next step reveals why the current one is important.
2. Remove the 7-segment display from PORTD.
3. Disconnect the decimal point form the 7-segment display on PORTB to free up a pin for later use.  (The remainder of this lab assumes that this is PB0).
   
4. Remove the photoresistor from the board and move the potentiometer to pin PA1.
   
5. Add the tri-color LED to your breadboard by connecting the Common Anode to Vcc, the red LED to pin 21 (use a 560Ω current-limiting resistor), the blue LED to pin 19 (use a 300Ω current limiting resistor), and the green LED to pin 18 (use a 300Ω current-limiting resistor). NOTE: To aid in debugging you may want to add 3 separate LEDs in parallel to your tri-color LED to see the light level of each color segment; however, make sure the parallel LED is separate (i.e. has its own current limiting resistor).
6. A cable has been provided for your accelerometer module, using black wire for ground, red for +5v, yellow wire 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).
7. Wire your accelerometer module into your breadboard. Attach the Y-axis output to ICP1 (pin 20).
8. 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.
9. 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 the accelerometer worksheet.

PART 2:

1. Read all of the directions for Part 2 before doing ANYTHING. Often the next step reveals why the current one is important.
2. Download the PWM sample code. It uses the output compare capability of timer 2 to generate a PWM signal to control the brightness of an LED connected to OC2 (pin 21). Modify the PWM sample so that you can control the LED brightness by adjusting the value of a potentiometer.
3. Implement a program that uses timer 1 input capture mode to determine the positive duty cycle. 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 50%.
   NOTES:
• You should choose as small a prescaler factor as possible for Timer 1 to increase the accuracy.
• 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 to restart at 0. The input capture interrupt does not reset TCNT1 to zero automatically.
  
4. Combine step 1 and 2 so that when you turn the accelerometer to the right it makes the LED brighter and when you turn it to the left it makes the LED dimmer. NOTE: If you turn the accelerometer-based virtual switch past 90 degrees it will start to decrease again. Use the one 7-segment LED display to output the most significant byte of your light level (the value of OCR2) in hex.

PART 3:

1. Read all of the directions for Part 3 before doing ANYTHING. Often the next step reveals why the current one is important.
2. You will need to manually generate three separate PWM signals to drive the tri-color LED. Use timer0 to generate 3 separate PWM signals for each segment. A period of <15ms should not be visible to a human. (For the end of the lab we encourage using a period less than 15 ms, but start at 15 ms and then reduce it as a smaller period will be more taxing on the processor.)
3. Use the potentiometer and the ATMega16's ADC to test your manual PWM signal. Use the ADC readings (0-255) to control the LEDs brightness. You might want to avoid using free-running mode to trigger the ADC as it will cause timing problems later. A good auto-trigger is Timer0 output compare (same interrupt that should be generating your PWM).

PART 4:

1. Read all of the directions for Part 4 before doing ANYTHING. Often the next step reveals why the current one is important.
2. Before doing any wiring for this part, make sure you have a program on the microcontroller in which pin PA0 is configured as an input in your program, PB0 is an output, and that PB0 is set to output low.  This will ensure that when you apply power to the board, the microcontroller is not trying to drive the output of the amplifier.
3. Refer to this schematic, which shows what you will need to add to make the field sensor work.  Note that it does not show components like the 7-segment display which should already be on your breadboard.
   
4. Obtain a TLC084 quad op-amp IC and 27 pF capacitor from the TAs.  The resistors you will need to add to your circuit can be found in the supply cabinet.
   
5. Add the TLC084 quad op-amp IC to an empty space on your breadboard near PORTA on the ATmega16.  Wire the chip to implement the amplifier circuit in the schematic, and connect the output of the amplifier to PA0 (pin 40.)  Move the potentiometer to PA1.  You may remove the photoresistor.  Try to keep your wiring especially neat for this section, since long wires could act as antennas.
6. Connect the transmit antenna to pin 1.  You will need to disconnect the decimal point on the 7-segment display to free up a pin on PORTB.
   
7. Connect the receive antenna to the input to the amplifier circuit.  Try to route your receive and transmit wires so they are not close to each other.
   
8. Apply power to the board.  Use a multimeter to measure the voltage between digital ground (the negative side of the power supply) and the analog ground (AGND on the schematic.)  You should see approximately 2.5 volts at this point.  Measure the voltages at the input and output of the amplifier circuit.  These should also be about 2.5V
   
9. Connect an oscilloscope probe to pin PA0.  (Make sure you connect the oscilloscope ground to the digital ground, not AGND.)  You should see an approximately flat line at about 2.5 V.  Now, touch the receive antenna.  You should see a lot of noise on the oscilloscope.
10. If you did not get the results from steps 7 and 8, something is wrong.  Check your circuit and wiring.  Do not proceed until your circuit passes these checks.
    

PART 5:

In this part, you will write code to utilize the sensor you just wired.  Read the following information, which describes the theory behind the sensor.  Then, complete the steps outlined in the implementation section.

IDEA:
The transmit and receive electrode are the plates of a capacitor.  Putting your hand in the vicinity of the capacitor plates changes the value of the capacitance.  So hand proximity will be inferred from capacitance changes.  We will be measuring small changes in capacitance, so we will need sensitive measurement techniques.  There is substantial noise (1/f or “flicker” noise, as well as 60Hz noise, etc) at low frequencies, so we will be modulating up to higher frequencies.  (Later we will also see that modulation can allow us to measure more than one channel simultaneously.)  Thus, we will measure the capacitance between the TX and RCV electrodes by looking at its response to an AC signal.
 
When we apply an AC voltage V=A cos(2 pi f t) to a capacitance C, a current I = C dV/dt is induced.  dV/dt = -2 pi f A sin(2 pi f t), so I = - C 2 pi f A sin (2 pi f t).  We will use the microcontroller to generate a square wave (instead of a sine wave) carrier.  The variable capacitance caused by hand motion amplitude modulates this carrier.  After some basic signal conditioning electronics on the receive side, the microcontroller will demodulate the received signal in software to recover a value proportional to C.

DESCRIPTION of SCHEMATIC:
The microcontroller will generate a square wave voltage on PORTB which is directly applied to the TX electrode.  This induces a current on the receive electrode that is proportional to the derivative of the transmitted voltage (and to the capacitance between TX and RCV, which we are trying to measure).  We are interested in the amplitude of the received signal, which is affected by hand position.  The derivative of a square wave is a train of positive- and negative-going spikes.  The first op-amp gain stage (U2A, one of 4 op-amps in the TLC084 package) is current amp (no input resistor, large gain 1M resistor in its feedback network) and also does some integration (the 27pF capacitor in the feedback network) to un-do the derivative and approximately restore the square wave.  The second gain stage (U2D) is a classic inverting (voltage) amplifier with a gain of 10 (100K/10K).  It simply amplifies the voltage output from the first stage.
 
The only remaining portion of the schematic provides the “analog ground.”  The voltage divider formed by R4 and R5 provides 2.5V that the single supply op-amps will use as “bias.”  The op-amps only have +5V and GND power supply connections (no negative voltage rail, like some op-amps).  So, a voltage signal of -2.5V will be represented by 0V; a voltage signal of 0V will be represented by 2.5V; and a voltage signal of +2.5V will be represented by +5V.

CODING:
Here is pseudo code for what you need to implement
 
// Set PORTB as output
// Set ADC0 as input; configure ADC
NSAMPS = 200; // Try different values of NSAMPS.  Look at the SNR / update rate trade off
Acc = 0; // acc should be a 16 bit variable
For (i=0; i<NSAMPS; i++) {
            SET PORTB HIGH
            Acc = Acc + ADCVALUE
            SET PORTB LOW
            Acc = Acc - ADCVALUE
}
Return Acc

IMPLEMENTATION:

1. Write a program that implements the pseudocode given above.
2. There are a number of tools that will be useful in debugging your circuit and program.

• The 7-segment display can be used to display a hex number, though if you update it too frequently, it may change too fast for you to read.  Displaying the highest 4 bits of a variable may be most useful.
• The JTAG debugger lets you view the values of registers and variables while your program is paused.  Set a breakpoint to occur after the sensor completes a measurement so that you can view the result of the measurement.
  
• The oscilloscope will allow you to look at the analog signals being transmitted and received.

PART 6:

1. Read all of the directions for Part 6 before doing ANYTHING. Often the next step reveals why the current one is important.
2. Combine the electric field sensor, potentiometer, and one axis of the accelerometer to each control one segment of the RGB LED.
3. The accelerometer should control the red channel, the potentiometer should control the green, and the field sensor should control the blue.  To verify that each sensor is working independently, it may help to have separate LEDs wired in parallel with the 3 colors in the RGB LED.  (See part 1.5.)
   
4. The electric field sensor will produce a wide range of values, which will depend on other objects that are nearby your breadboard.  You will need to use the JTAG debugger to determine this range of values each time you set up your board, and adjust constants in your code to scale these values to a range that is useful for controlling the PWM of the LED.
5. Timing is critical for the electric field sensor.  If an interrupt for something else happens during a reading from the sensor, it may affect the measurement.  Try to schedule events so that you read from the sensor during the time in between PWM interrupts.  Also, do not try to read from the field sensor and the potentiometer at the same time since there is only one ADC.
   

Extra Credit:

1. Implement a method of calibrating your sensor that removes the need to manually adjust constants in your code each time you set up the sensor.
2. Your method of doing this is up to you, but it should be intuitive and easy to use.  You may add another pushbutton to trigger calibration if you want.
3. After your calibration procedure has been performed, one should be able to adjust the LED brightness through its full range by moving a hand above the sensor.
   

Question 1: Describe why the code in your interrupt handlers could not be located in the main body of the program. Basically explain your design and why it was important for the specific code to be in an ISR.

Question 2: Explain why it is necessary to adjust the sensor each time it is moved to a new location or environment.

Question 3: Outline the timing of events in your program. When do your various interrupts occur in relation to each other? What timing decisions did you make to maximize the accuracy of the e-field sensor?

Question 4: Based on the lecture slides, briefly explain how the op-amp works. (i.e: how it works? what does it do? why do we need the op amp?)

Deliverables

Submit the files through Catalyst Tools at:

·         AA section

·         AB section

 

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

• Both partners' full name, login and student number.
• The lab number and the part of the lab (e.g. “Lab 3”).

1. Demonstrate part 6 to a TA. You can either do this during this lab, or during the first 1/2 hour of the next lab.
2. Turn in soft copy 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.