Sorry, what do you mean buy "register"?
If you want to generate a square wave tone at a known pitch/note, that's easy. Look at the [u]Play Melody Example[/u]. (There are charts on the Internet for converting notes to frequency.)
If you want to analyze a sound or signal and determine the note, that's tricky because of harmonics & overtones, and if there are chords or multiple sounds/instruments it gets even more tricky. If you want to try that search for 'FFT' (Fast Fourier Transform) or 'Autocorrelation'.