Most of the Mutable Instruments stuff is digital, based around STM32 microcontrollers. (Older modules - notably Branches and Grids - are Atmel based). All the sourcecode for MI modules is open source.
See also Ornament and Crime, based around a Teensy, and also entirely open source.
And of course, all the Monome modules such as Ansible are microcontroller based and open-source. (I point to open-source examples simply because there’s more proof than ‘look a microcontroller’). In fact, most digital modules and sequencers out there are built around a microcontroller - things like the Pamela’s Workout or Make Noise Rene.
This is also useful for clarifying: it’s the chip that is the microcontroller, not the board it’s on.
Personally, I’d advise against “bolting a devboard/Arduino onto the back of something” if manufacture is your goal; it’s a very expensive and phsyicially large way of achieving a goal. Arduino is very handy for sketching ideas in, for sure, but it’s not really the same as a production board. This approach does make more sense for DIY projects, as it exposes the hackability of the object nicely, and allows people to prod with things they might already have.
For instance, I’ve got one prototype that began on a bench as Arduino, got into my rack with nothing but a bare ATTiny14 on the back of it, and I’m now redesigning it all around a Silicon Labs EFM32 - the learning curve is slow but not vertical, but the path to manufacture - around a single QFP48 chip and some caps - is much clearer, and the capacity of the board much higher.
If you’re interested in generating CV, O_C is a good thing to look at - it’s using a 3.3V microcontroller to control DACs spitting out higher voltages, which is going to be a key part of CV generation.
