I ran through some demos and then set it aside. A year ago I started using is a logic analyzer for my Atari cartridge interface project. Recently, I decided to finally load up the Atari800 core I'd read about. I didn't do much research when I bought the board in 2015. Had I understood more, I probably would have bought a MIST, although the Papilio was about half the price I imagine. But I digress.
Thankfully, 64KiB.com (aka foft) ported the Atari FPGA implementation to the Papilio DUO. It loaded just fine, but it acted like the down arrow on the keyboard was stuck. Fortunately, the Papilio and 64KiB creators had already solved this problem.The AVR needed to be programmed for high-impedance inputs on all GPIOs. Whew.
Another problem I ran into is keyboard mapping. I have a US-layout PS2 keyboard, which has a different layout of where the Atari arrow keys go compared to a UK-layout. I could only make the Atari cursor go in 3 directions. So RTFM and I learned about the UK-layout problem and started randomly trying CTRL-key combos. "CTRL-\" worked for me.
I played my GRAVITEN game, which was the second I wrote for the BASIC game competition. Fun.