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Computer Science 230 Assignment 2

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Computer Science 230
Assignment 2

Objectives
• More practice writing assembly language programs
• Introduction to peripherals: LEDs and buttons
• Introduction to subroutines
Academic Integrity
Prior to submitting your assignment, you should familiarize yourself with the University policy on
Academic Integrity
We will use a plagiarism detection tool on all assignment submissions.
AVR Studio Project
As in assignment 1, you should create a new assembly language project in AVR Studio for this
assignment. Download the sample code and add the files to the project.
Subroutines
In the sample code, subroutines use specific registers to pass parameters and return values. Registers
are effectively global variables, so register usage must be consistent across every subroutine. The
register usage is documented in the comments.
Rather than tracking register usage by hand, you may wish to update the subroutines to use the stack to
save and restore registers. If you do this, be sure to initialize the stack pointer and update the
comments appropriately regarding register usage.
Part I – Lighting the LEDs
As you’ve seen in the lab, the boards include a 6 LED strip, where each LED can be controlled
independently from the others.
Although the LEDs are wired to adjacent Arduino pins, they are controlled using two AVR ports:
PORTB and PORTL. The exact mapping is shown in the table below:
Pin PORT Bit #
42 L 7
44 L 5
46 L 3
48 L 1
50 B 3
52 B 1
You should modify the code in a2q1.asm so that it displays the value in R0 in binary using the 6
LEDs. You should consider the LED on Pin 42 to be the least-significant bit and the LED on Pin 52 to
be the most-significant bit.
For example, if R0 contains the value 0x13, the LEDs should look like:
Pin 52 50 48 46 44 42
Value OFF ON OFF OFF ON ON
Be sure to test your code on several different values.
Your instructor’s solution used a combination of ANDI and BRNE instructions to test the individual
bits in R0 and then used ORI to set individual bits in two output registers: one that is output to PORTL
and another that is output to PORTB.
Be sure you understand the assignment 2 exercise posted with the assignment before you start
programming. It will be the in class exercise on October 12, 2018.
Part II – Subroutines and timing
Using the code you wrote in Part I and the delay code provided to you in Lab 4, write a program that
does the following:
counter = 0
start:
display counter in binary on the LEDs
counter = counter + 1
delay between 100 and 300 milliseconds
goto start
First you should turn the code you wrote in Part I into a subroutine called display. Then write a main
program that implements the algorithm shown above. It will be easiest if you use r0 for your counter.
Part III – Button press counting
Using the button code and delay code provided to you in Lab 4, write a program that does the
following:
counter = 0
start:
if button pressed
counter = counter + 1
delay between 100 and 300 milliseconds
display counter in binary on the LEDs
goto start
Part IV – Improved button code
Improve the button code so that it correctly determines which button has been pressed. Study the
comments in a2q4.asm for details on the values returned from the analog to digital conversion when
the buttons are pressed.
You’ve been provided with a main program – you’ll need to fix the button subroutine and include your
LED display subroutine. Once you do that, a different light should come on for each button you press.
Your instructor’s solution used the BRSH instruction.
Submission
Submit your a2q1.asm, a2q2.asm, a2q3.asm and a2q4.asm using connex. Do NOT submit
your project file – just the .asm files.
Grading
If you submit a program that does not assemble you will receive 0 for that part of the assignment.
Question 1 4 marks
Question 2 4 marks
Question 3 4 marks
Question 4 2 marks
Total 14 marks

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