A bit of history:
A while back I made a soundchip that interfaces with the popular graphics calculator: TI84+. I dubbed it TIMID. This is a combination of TI+MIDI+SID, because this chip can relay midi messages from the calculator and it emulates a SID. Because this is not a real soundchip but an emulated one running on a microcontroller I can use excess cycles of the microcontroller for other things like general purpose I/O and reading data from a DS touchscreen (not implemented yet). This project is not finished yet. I have been working on this project for about a year and a half now. 1.5 years ago back when there was not a lot of information on interfacing hardware with the calculator I was destined to make a device that reads and sends data respectively from and to the calculator and interacts with the world. This started out as a profile essay for my highschool grew out to a fullfledged sound interface with some neat features such as a built in lookup table for musical notes and midi output.
Here is a video for those who don't like reading.
http://www.youtube.com/watch?v=R_bKXSzpjes
Features:
-Two wire interface
-Backwards compatible with TI83+
-SID emulation
-MIDI out (relays midi messages from the calculator)
-Source code written for Atmega328 can be used with most arduino compatible boards.
(I will release the full source when there is a stable version that does not destroy calculators nor audio equipment. As of right now the PWM output might damage audio equipment because of DC offset of almost 5v)
The SID library I used is (c)2007 Christoph Haberer. I changed it for my own needs though.