CSE466 Lab 2: “Interrupts”

Objectives

The goal of this lab is to introduce various features and modules of the ATmega16 while introducing how to program the microcontroller using C. In this lab you will learn the following:

• how to operate an oscilloscope;
• how to program a timer interrupt;
• how to debounce a button in software;
• how to use the A/D converter and its interrupt features.

This lab assignment has 4 required sections. Some of the sections have multiple graded parts so make sure to save ALL the graded parts so they can be turned-in and/or shown to the TAs for grading. The deliverables section below outlines exactly what will be graded.

Section A: Interrupts in Assembly

1. Implement your “One Minute Counter” from Lab 1 (in assembly) using a timer interrupt to control counting rate instead of a delay loop. (GRADED DEMO DUE THIS WEEK)

Section B: Introduction & Tutorial

1. Add additional hardware to your breadboard

2. Complete the Oscilloscope tutorial

3. Complete the JTAG debugger tutorial.

Section C: Interrupt Timer in C

1. Reimplement the counter in C. In addition, the counter should also be able to start, stop, and change direction based on the two added buttons. (GRADED DEMO)

Section D: Analog to Digital Converter

1. Read the analog value coming from the potentiometer or photoresistor and output the most significant 8-bits in hexadecimal notation to the two 7-segment LED displays. Use a button to toggle between displaying ADC values from the potentiometer and the photoresistor. (GRADED DEMO)

2. Use the potentiometer to create an adjustable light sensor that turns on an LED when the light level drops below the value of the potentiometer. (GRADED DEMO)

Suggested Reading / Helpful Hints

• The following sections of the ATmega16 datasheet might be helpful for this lab (listed in table of contents).
• Interrupts
• 8-bit Timer2
• Analog to Digital Converter
• Here is a reference on bypass capacitors, which are used to reduce noise in your circuit: http://www.seattlerobotics.org/encoder/jun97/basics.html.
• Even though the ATMega16 is an 8-bit processor, an int is 16 bits.
• Debouncing a button is essentially an averaging or smoothing of the input value. This is often referred to as low-pass filtering because only the long-term variations (at low frequency) are used (passed) and rapid changes (at high frequency) are discarded. A good way to do this in software is to sample a button several times and take the average or mode (the most common value across all the samples – this is like voting for Boolean values).

Resources

Lab 2 Schematic

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

Suggested Steps

Section A: Interrupts in Assembly

1. Reimplement the 1Hz counter by eliminating the delay loop and replacing it with code that uses the output compare interrupt from timer2 to do the timing. Documentation on the interrupt jump table and interrupt handlers can be found in the datasheet. Remember, when writing the interrupt handler routine, you'll want to minimize the amount of code in it. Interrupt handlers are meant to be very fast, and do only the work that is critical to do right at that instant. There is no way that anyone will be able to notice if your LEDs update even a few milliseconds late, so the LED update code should not be in the handler. Documentation on timer2 including all the related registers is available in the datasheet.

Question 1: Why do the Reset handler jump and Timer 2 interrupt handler jump need to be located at a specific address?

Section B: Introduction & Tutorial

1. Add two buttons (pins 28 and 29), a potentiometer (pin 40), a LED (sharing pin 29 with one of the buttons), a wire to Vcc (pin 32), a photoresistor (pin 39) and bypass capacitors to complete the circuit shown in the schematic on your breadboard. Make sure not to forget the current-limiting resistor for the new LED, and pull-up resistors for the two buttons. Pay special attention to how the new LED is wired. It shares a pin with one of the buttons. We can use this pin for both button-sensing and driving the LED by controlling whether the pin is an input or an output. Again, before powering your circuit back up, you should confirm that you don't have any shorts between Vcc and ground with the multimeter.

Question 2: What is the appropriate range of values for the two button pull-up resistors? (Refer to datasheet)

Question 3: What is the purpose of the bypass capacitors? Why might they be useful for this lab?

2. Go through the entire Oscilloscope Tutorial (consult the Oscilloscope Manual as needed).

Section C: Interrupt Timer in C

1. The recommended program for development for this class AVR Studio. We have provided a sample project that will copy a constant from program memory and displays it on PORTB. When the sample program is uploaded to your breadboard an “8” should appear on the 7-segment display attached to PORTB.
2. Rewrite your interrupt timer using C. Documentation for the avr-gcc compiler is available at C:\WinAVR\doc\avr-libc\avr-libc-user-manual-1.4.4.pdf. From that link, go to the modules section and then the Interrupts and Signals entry. You will be using the same interrupts as before.
3. Implement the stop-start button so that pressing the button will cause the counter to toggle between counting or halting. The left (tens digit) decimal point should indicate whether the counter is running by being on when the counter is running and off when the counter is stopped. It would probably be a good first step to get the switch debounced (see the hint on debouncing). You can either use timer0 for debouncing or figure out a way to use timer2 for both the counting and debouncing. Remember that if you push the button and hold it for a long time, it should still only register as a single push.
4. Implement the up-down button so pressing the button will change the direction of counting. The decimal point on the right (ones digit) should be on when counting up and be off when counting down. Note that even when the counter is stopped, the system should still process an up-down button press by immediately updating the direction and decimal point state.

Question 4: Why debounce a button?

Section D: ADC 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.

1. Create a hexadecimal display that can properly output an 8-bit value on the two 7-segment LED displays that combine to make the 8-bit value (one 7-segment LED will represent the high 4-bit nibble and other will represent the low 4-bit nibble). You should implement a lookup table in program memory to convert a 4-bit nibble (in binary) to a hexadecimal number. The hexadecimal letters should all be uppercase except for the letters ‘b’ and ‘d’ which should be lowercase to avoid repeated symbols. The easiest way to ensure that you can properly output all of the 8-bit values properly in hex is to create a simple hexadecimal counter that counts from 00 to FF. (Hint: Displaying an 8-bit value will be useful this quarter as a debugging mechanism so the code will most likely be reused. i.e. it might be helpful if you write a function that takes an int from 0-255 as input and displays 00-ff on the LED displays.)
2. Implement a program that continually displays the analog value of the potentiometer on the two 7-segment LED displays. You should read the section in the datasheet on the Analog to Digital Converter to find out how to obtain the analog value. You'll want to use the interrupts to tell you when the conversion is complete. Also note that the ADC has 10-bit resolution but only 8-bits are needed for the display, so you can throw away the 2 least significant bits. Note that the datasheet describes a nice way to get the 8 most significant bits.
3. Use the button on pin 28 to toggle between the system displaying 8-bit analog values from the potentiometer or the photoresistor. The two 7-segment displays should either show the 8-bit value of the potentiometer (decimal points off) or photoresistor (decimal points on).

Question 5: Does brighter light or dimmer light give you a higher analog value? What is happening to the resistance?

PART 2:

2. Read all of the directions for Part 2 before doing ANYTHING. Often the next step reveals why the current one is important.

1. Use the potentiometer and photoresistor to create an adjustable light sensor that will turn lights on when it becomes dark. The photoresistor will be used to monitor the current light level of the surrounding environment. The potentiometer will be used to adjust the level at which the sensor will trigger the lights to come on. This will allow the user to customize their home so the lights turn on and off at the desired darkness. Once the photoresistor reports that the level of darkness specified by the potentiometer has been reached, the ATmega16 should turn on the lights. (In this case the light is an LED attached to pin 29.) Once it becomes light enough the ATmega16 should turn off the LED.
2. To determine the voltage difference between the potentiometer and photoresistor you should use an ADC differential channel. The potentiometer should be the positive differential input and the photoresistor should be the negative differential input. The ADC will return a signed number (in two’s complement) that represents the voltage difference between the two differential input channels. The two 7-segment displays should display this signed 8-bit value by using the tens digit decimal point to represent the negative sign.

NOTE: You might want to average some of the ADC values to prevent the light from flickering on and off. A weighted moving average might be a good idea for smoothing. Below is an example formula for a weighted moving average where a larger value for x will cause “new value” to have a smaller affect on the average.

average = ((previous average * (x -1)) + new value) / x

Note: The atmega chip does not have hardware to handle floating point numbers. DO NOT USE THEM.

3. If we want to lower the power consumption of our light sensor we may want to set a slower sample rate. We can lower CPU utilization by using a timer to wake-up the processor to trigger an ADC sample. You can safely assume that updating the light state (on or off) can take place a second after the ambient light level has changed. This means that you should design your system to sample at a reasonable rate so that it would be possible to use less power.(NOTE: You are NOT required to put it into a low power or sleep mode). If you sample approximately every 100-500 ms (depending on the amount of smoothing) your system should still be sensitive to changing light levels. For this lab you need to sample the ADC on the order of milliseconds.

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 2, Section A, Part 1” for your assembly interrupt counter).

Answers to the Five Lab Questions.

Section A:

1. Turn in a soft copy of your commented assembly code. You need to demo this during your lab section.
• Your counter should be functionally equivalent to the lab 1 final counter.

Section C:

1. Demonstrate your counter to a TA. You can either do this during this lab, or during the first 1/2 hour of the next lab.
• It should have a reasonable user-interface. (i.e. make sure buttons are responsive but not overly sensitive).
• Changing counting direction should work properly and the decimal points should represent the current state of the counter. The low decimal point should be lit when counting upwards, and off when counting downwards. The high decimal point should be lit when the counter is running and off when the counter is stopped. The low decimal point should still change state even when counting is stopped but the up-down button is pressed.
• Counting should happen at 1 Hz.
2. Turn in soft copy of your commented C code.
• Make sure that your interrupt handler is as minimal as possible. Points will be docked for updating the displays inside of the interrupt handler.

Section D, Part 1:

1. Demonstrate your light/potentiometer reader to a TA. You can either do this during this lab, or during the first 1/2 hour of the next lab.
• The display should be able to range from very close to 0 when the pot is turned all the way one way to very close to 0xff when the pot is turned the other way.
• The decimal point should show whether the values being displayed are from the potentiometer or the photoresistor.
• It should be reasonably responsive to changes of the potentiometer or in the level of light.
  
  
2. Turn in soft copy of your commented C code.

Section D, Part 2:

1. Demonstrate your light sensor to a TA. You can either do this during this lab, or during the first 1/2 hour of the next lab.

• The decimal point should show whether the value being displayed is positive or negative.
• The LED should not flicker excessively and the readings should be somewhat stable when the surrounding light is staying constant.

2. Turn in soft copy of your commented C code.

• Should sample the ADC on the order of milliseconds