A couple years ago I was looking for a RPN calculator like the old HP calculators from my youth. Didn't have a lot of luck, so I made my own..
Its running a teensy 3.2 processor driving an Adafruit touch screen along with an Adafruit LiPo battery charge controller & battery. It doesn't have the huge accuracy of the original machines, but its good enough for my use.
Here's a video of it running.
And the story is here : The calculator project. Note : This is very out of date See The Left Coast Handheld for more current info.
There are links to the code on the webpage. The code is somewhat old and out of date. Will it still compile and run? Not sure. The actual calculator heart is one stand alone object file and completely cross platform. It was originally developed on my Mac. It takes key presses as text and sets the registers for you to display.
Enjoy!
-jim lee