GetLoFi posted detailed info on using their LTC1799 Precision Oscillator kit for pitch control on a Casio SA-39 Keyboard -
The kit oscillates at frequencies from 1KHz to 30Mhz and its output frequency is controlled with a potentiometer. The oscillation frequency from this kit can be substituted for the constant oscillation frequency a device receives from its internal crystal. The kit’s variable frequency will allow you to control the pitch of a device when it is patched in place of –or sometimes in parallel with—the device’s internal crystal.
Likely interesting even if you aren’t working with that specific hardware – Circuit Bending Casio SA-39
The LTC1799 is a very interesting part — one that I hadn’t seen before. It’s absolutely tiny, the size of a surface-mount transistor (SOT-23), and can do many of the things that a 555 can do. In fact it’s a better chip than the 555 in some ways, although it’s only specified to work down to 1kHz in frequency. Imagine a Drawdio built using this chip: so small that the active components are dwarfed by the battery and speaker. Of course, that makes it harder to solder!