Pins and Routing

The Arduino Mega has a heap of pins. But will it be enough? In order to get some idea I've begun to try and enumerate the assignments of the pins. Here's a screenshot of the color coded spreadsheet:

So far it's looking fine, like there'll be enough to go around and some to spare. The actual spreadsheet is available here.  There's a second sheet that lists the pins in numeric order.

Unfortunately listing the pins is one thing, actually routing them on a PCB to where they need to go may be a totally different matter altogether. I've been tossing up how to to handle this. One way would be to make each module a conventional Arduino shield. I'd need to have an additional bus to transfer the audio and modulation signals. The advantage of this is that the routing and component layout would be fairly easy and it would be a neat stack of boards with a very small footprint. The downside is that it would be fairly expensive as a board the size of an Arduino Mega for each module would be quite large. I could add more than one module per shield but that's starting to work against the modular nature of the design. So in that case I think it would make better sense to have a single shield and have the modules attached to that. Something like:

I'm not sure exactly how the LCD screen and the rotary encoders would be arranged but they, along with the mixer and modulation router, would be on the main shield. This shield would overlap the Arduino Mega a little and have pin attachment points for the various modules. This will allow the module boards to be only constrained by size in one dimension - where they mate with the main shield and each one can be considerable smaller than a shield itself.  However the routing of all the pins becomes a lot more complicated. One thing that might make that easier is to make this one shield a four-layer board. I've never laid out a four layer board but it sounds like KiCad can handle it OK and a four-layer board is only twice the price of a two layer one with OSH/Park. Spending a bit more on this one shield will pay off in allowing the module boards to be smaller. 

At the same time I thought about it and realised it likely makes sense to have three oscillators. This was partly from looking at the pins available, partly because the Mega can sustain three Timers and partly because OSH/Park has a minimum order of three so it costs me no more to have three identical waveshapers than two from that point of view. The block diagram now looks like: