Assignment 1: Implement a Larson Scanner

Written by Pat Hanrahan

Due: Tuesday, April 18, 2017 at 5:00 PM

Glen A. Larson, the man responsible for producing the original Battlestar Galactica and Knight Rider television series, often used simple lighting effects in his shows. One famous example is a scanner that consists of a set of red LEDs that scan back and forth. The front of the Knight Rider car had a scanner.

Knight Rider Car

Check out this short video tribute to Glen Larson. How many times does the scanner pattern appear?

Check out the Larson Scanner Kit Demo from

When you’re done with this assignment, you should

Basic part: simple scan pattern

This assignment involves modifying the blink program showed in class. blink.s is an ARM assembly language program that blinks a single LED. To implement a scanner, you will need to control at least 4 LEDs.

  1. Wire up your breadboard with 4 LEDs. If you feel ambitious, and want to make more interesting patterns, wire up 8 LEDs. Make sure all the LEDs on your breadboard work.

  2. Connect the LEDs to GPIO 20-23 (or GPIO 20-27, if you use 8 LEDs). The leftmost LED in your scanner should be connected to GPIO 20, the second from the left to GPIO 21, and so on, such that the rightmost LED in your scanner is connected to GPIO 23 (or 27 if using 8 LEDs).

  3. To get the starter code, clone the assign1 branch from your 107e GitHub assignment repo, like you did assign0 earlier:

    $ git clone -b assign1[YOUR-GITHUB-USERNAME]-assignments assign1
  4. Right now, larson.s in the starter code is simply a copy of blink.s from lab. Add code to scan back and forth from the pin-20 LED to the pin-23 (or pin-27) LED. Exactly one LED should be on at any given time, so it looks like the light is ‘moving’ between LEDs.

    You will first need to initialize all the GPIO pins. Then you will need to sequentially turn LEDs on and off.

    Make sure you handle the left and right sides properly (that is, the left-most and right-most LED should be each on for the same amount of time that a middle LED would be on). You should write the program without any function calls (because we haven’t taught you how to create and call functions in assembly language yet). Although you cannot use functions, you can still strive for simplicity by using some clever loops. We will be grading your code quality, so try to block out your code, pick good label names, and add comments where necessary.

    A rule of thumb for style is that if your code is well-designed, it should take no additional code to extend it from 4 to 8 LEDs. If you find a lot of copying and pasting in your solution, that’s a bad sign!

    Note that we have also included a file called Makefile in the starter code. This script is similar to the doit scripts you saw in class—it will use the as and objcopy utilities to compile your larson.s into a larson.bin file that you can install to the Pi. To execute the Makefile, you should run make from the command line in the directory containing the Makefile, e.g.

    $ cd assign1
    $ make
    $ larson.bin

    For now, don’t worry about how it works. We’ll go through Makefiles in-depth in the next lab.

  5. Generate the scanner pattern. Enjoy watching it late at night in your room. Impress your friends.

Extension (Do at least 3 of these during the quarter for an A)

(See the assignment policy page for more about our grade expectations.)

The basic part of this assignment should not take too long once you understand ARM assembly.

If you want to explore further, extend the program. The extension is more challenging, and may involve using additional ARM assembly instructions.

If you watch the Larson scanner carefully, several LEDs are turned on at once. The center one is at full brightness, and the ones on the edge are dimmer. How would you dim an LED? Implement a scanner with LEDs dimmed in a way that would please Glen Larson.

You should have a central, full-brightness light that scans just like in the basic version of the assignment, but you should visibly dim the neighbors of that central light as it moves. It should look similar to the Larson Scanner Kit Demo.

Try to have at least three distinct levels of brightness.

If you choose to implement the extension, just submit that implementation as your larson.s in your repo: you don’t need to also submit an implementation of the basic assignment. You should get the basic assignment working before you start on the extension, though!


The automated check here is more likely to fail than in assignment 0, since there are more things that can go wrong. In this assignment, the automated checker is seeing that:

When we actually grade your submission, the human grader will assemble the larson.bin file again, then test it on a physical Raspberry Pi with the LEDs plugged in.

As stated on the assignment policy page, if you submit an assignment solution that does not compile on CI, we will deduct 1 point from your basic grade.

See the automated checker guide for more information about the system, and details on how to see what went wrong.