About the circuit:

• A day after I ordered the PCBs I realized I had forgotten to put a power switch on the board. I thought maybe using sleep modes and wake-ups by interrupts I could solve that problem but to my disappointment the MKL25Z only has interrupts in ports A and D, and I connected the keypad to B and C. Because of this the calculator powers on and off putting in and taking the battery out (sighs).
• I also forgot to design a reverse voltage protection circuit, which combined with the previous problem becomes a great danger.
• If you want to replicate the PCB please make sure that the silkscreen doesn't overlap the SWD programming interface (very important)

About the code:

The library used to drive the LCD was made by [imachooon]. Thank you for your library [imachooon].

The code is not finished at the time of writing (and probably won't be for a long time) and can present some bugs, but it works.