CSEP567 Lab 3: “Counting in C”

Objectives

The goal of this lab is to introduce 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;  

Reading / Helpful Hints

• The following sections of the ATmega16 datasheet might be helpful for this lab
  • Interrupts
  • 8-bit Timer0
  • Analog to Digital Converter
    
• Here is a reference on bypassing, a technique 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

• www.avrfreaks.net 
• Brief introduction to AVR Programming
• avr-gcc manual
• Application notes section for the AVR 8-bit RISC family

Suggested Steps

Section A: Introduction & Tutorial

1. The supported program for development for this class is Programmers’ Notepad. 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. If you would like more information on the makefile, programmers notepad, and winAVR please refer to install_config_WinAVR.pdf .  To make Programmers’ Notepad easy to use in this class the lab staff has added 3 useful commands to the tools menu:

•   “Tools -> Make All” – build all the possible output files for the project
•   “Tools -> Make Clean” – delete all output files from the project
•   “Tools -> Program” – launch MegaLoad

2. Go through the entire Oscilloscope Tutorial (consult the Oscilloscope Manual as needed)
3. Add two buttons (pins 18 and 19), a potentiometer (pin 40), a LED (pin 21), a wire to Vcc (pin 32), a photoresistor (pin 39) and bypass capacitors to complete the circuit shown in the schematic on your breadboard. Again, before powering your circuit back up, you should confirm that you don't have any shorts between Vcc and ground with the multimeter.

Section B: One Minute Timer

1. Implement a counter in C that counts at 1Hz from '00' to '59' using Timer 1's output compare interrupt for timing. Documentation for the avr-gcc compiler is available at http://www.nongnu.org/avr-libc/user-manual/index.html   From that link, go to the modules section and then the Interrupts and Signals entry. 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.
2. Implement the stop-start button (pin 18) 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. We have provided some sample code in the prelab for debouncing a button. You should setup the timer to sample the button at 5ms intervals. Remember that if you push the button and hold it for a long time it should only register as a single push.
3. Implement the up-down button (pin 19) 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. We have provided some sample code in the prelab for debouncing a button. 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.

Demonstrate your One Minute Timer to a TA. Make sure you have debounced both buttons and implemented the specification listed in Section B.

Question 1: Why debounce a button?

Section C: Hex Counter

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. (NOTE: Displaying an 8-bit value will be useful this quarter as a debugging mechanism so the code will most likely be reused.)
2. Implement a counter in C that counts at 1Hz from '00' to 'FF' using Timer 1's output compare interrupt for timing.

Demonstrate your Hex Counter to a TA.

Delieverables

Turn in a copy of your commented C code from Part B and your answer to the question.