I built an ATmega328 in my calculator once and the reset line was tied to the ON button. It works fine. How are you going to wire up the usb? You cannot put more than two devices on the serial bus. I assume you pulled the charger and regulator circuits from the datasheet of those IC's, so I can safely assume those are allright. I'd make a prototype first before you order a pcb. With some smd prototyping board you could maybe even fit it inside a calculator.
At the moment I have some hardcoded patches that you can choose from, but in the future you will be able to store custom patches in the microcontroller's memory. There are no knobs on the synth and there are no plans for adding them in the future. I pretty much used up all I/O pins on the microcontroller.
Yeah that is why I made this too. I made a soundchip for my calculator using the same microcontroller I am using for this project, but the emulation does not sound close enough to the original. It is a nice soundchip emulator for a calculator, but it cannot play back all sid tunes correctly. This thing however can play back anything in theory.
Soldered it up and made a little enclosure for it. It has an audio jack, 12Vdc jack, midi jack, DB9 serial rs232, a power and midi LED and a switch for switching between the rs232 and midi port.
IC's used: MOS6581 SID soundchip ATmega328 (microcontroller used in the arduino uno) PC111 optocoupler TL072 Op-amp 7805 voltage regulator 7812 voltage regulator MAX232 serial level shifter
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