If you are interested in making a very basic scope to monitor some simple things before you spend the money on a real scope have a look at the work that vaupell posted in the Element 14 community. He tried a number of internet scope projects until he found this one that worked like a charm.
“After trying many different versions of code and tutorials, I was unable to get a single one to work, and all the tutorials and guides around was 2-3 years old. Not sure if it is the IDE or the actual hardware that has changed in such a way that it didn’t work anymore. I finally found a working oscilloscope from a Japanese website, (linked below) and a working TFT screen library, meaning i could read the various signals received. Clearly the limitations are 0-5v but that is fine, what i needed to know is either wave type and frequency, and this is sufficient. It also gives me an indication of the amplitude which is just a nice bonus.”