Re: Teensy 3.5/3.6 upgrade for uBITX
The uBITX was designed to use the pre-existing Raduino board. My opinion on a fresh start would be to put the Si5351A directly on the uBITX board, so only the I2C digital pins would need to be routed. This would leave a cleaner option to connect various micro-controller boards, depending on the features desired and budget. Separate the RF analog board from the digital / audio processing board. But we are not limited to what we can do, so no real issue here.
IMO, the Teensy 3.6 is a wise choice for adding DSP and SDR features with the option to stay in the familiar Arduino IDE. It appears that PJRC's business model is willing to cover their code development cost with the sale of the boot loader code pre-flashed in the MKL02/MKL04 chip ($7 US) and very generous IMO. This means there's an option to also place the K66 directly on your own board design and just buy the boot loader IC from PJRC. PJRC is a business, we are mostly just here as a hobby. If we weren't hobbyists or working to enable hobbyists, we would probably not be using Arduino IDE :)
I am an advocate of the Teensy/K66 direction, but last week I ordered a Protoneer Nano-Arm. https://wiki.protoneer.co.nz/NANO-ARM $10 + $5 shipping to US from NZ. This board has the same footprint as the Nano, and _should_ be a fairly easy Nano hardware replacement directly on the Radunio board. (might need to change out the 5V reg to 3.3V, etc) This may be an interesting option for those who would like a micro-controller modernization / upgrade without the budget or desire to move up to all the options Teensy /K66 enables. One one hand, I want all the features. One the other hand, I wish to stay true to Farhan's original goal of a $100 radio shipped, or take it to the next level of minimizing total system cost.