Description
Background
This assignment is a variation of the one created by Dr. Leonard Manzara for CPSC 359, Winter 2019.
In this assignment you will use the Raspberry Pi (RPi) frame buffer to create a visual display. You will also use the
GPIO capabilities of the Pi to operate an external device, a Super Nintendo Entertainment System (SNES) controller
(https://en.wikipedia.org/wiki/Super_Nintendo_Entertainment_System).
Objective
Your goal is to write a program, in C, that emulates an Etch A Sketch, a retro toy from the 1960s (https://en.
wikipedia.org/wiki/Etch_A_Sketch). Etch A Sketch allowed people to make rudimentary drawings with
a small set of simple functions. Although Etch A Sketch was a mechanical device, you can emulate its functions with
software on the RPi.
Etch A Sketch had three controls summarized in Table 1. Note that the Etch A Sketch was like a plot that could not
lift its pen. So from a default starting position, the user moves the “pen” up/down/left/right to draw a picture. The user
can restart at any time by inverting the Etch A Sketch and shaking it to erase. After erasing drawing continues from the
last position of the pen.
Etch A Sketch Control Description SNES Implementation
Horizontal knob Move drawing position left and right Left and right joystick buttons
Vertical knob Move drawing position up and down Up and down joystick buttons
Invert/shake Erase image Start button
Table 1: Summary of Etch A Sketch controls.
Details/Hints
You may use code from examples provided in the course material through D2L – but be sure to note in our source code
comments where the code came from.
You should use the system timer counter register to time your delays. See the Broadcom ARM Peripherals manual,
p 172. It is not more efficient than than using delay loops, but it does expose you to another, easy-to-use capability of
the Raspberry Pi. You are not required to use interrupts for this assignment.
You may notice that writing pixels to the frame buffer is slow – it takes almost one second to fill the screen. Don’t
worry about it. It is beyond the scope of this course to fix the problem.
Although not strictly necessary for this assignment, I found it helpful to implement a line drawing function. You
may want to look at Bresenham’s line algorithm. If you use an implementation that you find online, be sure to cite the
source.
As stated above, use the C language because: it will give you more practice with an important programming language, and it is much easier to work with then assembly language. In fact, this kind of low-level machine programming
is precisely what C does best. See https://en.wikipedia.org/wiki/C_(programming_language).
As with any programming project, I recommend that you approach this incrementally. Start by implementing a
simple subset of the required functionality, then add the remaining functionality one piece at a time. I suggest doing
the interrupts last.
Debugging is hard to do in this bare metal environment. Doing incremental changes will help you to zero on the
source of errors. Also, UART output may help to find some bugs, so set that up.
Good coding practices include all the good habits you should exhibit when writing your code. These include, but
are not limited to:
• indentation,
• informative variable and function names,
• helpful comments,
• the presence additional, helpful files such as readme.txt and Makefile, and
• code modularity.
Deliverables
Turn in the following items for evaluation.
1. All files required to build your program. This includes all of your source code files, Makefile, and a readme.txt
file with instructions.
2. A demo of your working program for your TA during a tutorial session – date to be determined by your TA.
Bonus (10%)
For a 10% bonus, you can implement a flood fill algorithm. I used the SNES X button to initiate the fill. This should
be a fairly obvious algorithm if you use what you learned in CPSC 331, but if you look anything up, be sure to cite
you sources.
Evaluation
1 Program runs as specified
Drawing controls work 5pts
Erase works 5pts
2 Configuration of I/O pins 3pts
3 Configuration of frame buffer 4pts
4 Coding practices 3pts
5 Flood fill (bonus) 2pts
Total 20pts
There is no team work. Each students must submit individual solutions.
Late work
After the deadline and up to 24hrs late: -5pts. After 24hrs and up to 48hrs late: -10pts. Over 48hrs late: -20pts, i.e.,
no assignment will be accepted beyond 48hrs after the deadline.
Plagiarism
Work must me the sole work of the individual submitting the work.
You may find that the this assignment is similar to other published code. Nevertheless, you should go through the
process yourself, and submit your own work. If you do borrow from other sources, cite the source and clearly indicate
what you have borrowed, keeping in mind the design must be substantially your own. If you cite your sources, worst
case you may receive a reduced grade for borrowing too much. If you borrow, but do not cite, that is plagiarism
and academic misconduct, and carries severe penalties as determined by the Faculty of Science. You may use code
provided in the course content on D2L, but you must still cite the source.
As a guideline, consider the 20-minute rule. Talk with your colleagues and consult other sources (cite them please).
Wait at least 20 minutes, then do your work to be sure that it is your own. Less then 20 minutes usually means that
you are merely copying work from the original source.
