I have been contemplating making a handheld board that simulates/emulates a PDP-8, but with an octal control panel so you can key in instructions and step through them and watch how they affect the accumulator, etc. I also want it to be able to run a lot of the PDP-8 software such as OS/8, etc. I've played around with some PDP-8 emulation and what I've always loved about the machine is its simplicity. So, back to the idea of making one, at first I was thinking it would be workable to use some SPI FRAM for the memory because it needs 32768 12-bit words of memory, certainly more than any AVR has. After considering the speed of serial memory, even if I run it at almost 8M, it is still going to take 5us or so to load or save a value and the PDP-8 instruction speed was 1.2us total so I am scrapping the idea. That leaves me with two choices, either abandon the idea of doing it with an AVR and go to a STM32 with enough built in SRAM or use parallel memory (SRAM, FRAM, etc.) with an AVR. I've done a little STM32 work with Atollic TrueStudio and while it was workable, it is a whole different world. I prefer the simplicity of AVR's. I also plan to drive 9 seven segment displays, a row of LED's, and scan two keyboard rows, so 8 columns + 12 rows and 20 pins are gone there. 5 pins for the microSD with card detection. 4 pins for two UART's. A 64 pin could probably be done with a couple of latches for the memory (16 addr, 8 data minimum, or 15 addr, 12 data for a word at a time). My experience is that I'd almost always rather just go to the 100 pins and not deal with the latches though. So I am looking at these two and here is my comparison between them so far:
flash sram price speed XMEGA128A1U 128 8k 6.26 32 MEGA640 64 8k 8.16 10.6 MEGA1280 128 8k 10.95 10.6
Speed, could do a 29.4912 MHz instead of 9.216 MHz (3.2x faster)
110 baud capable because of baud scaling.
Memory interface, but would have to do 12 bits in two 8 bit passes, still easier though, and likely faster.
Stronger pin drive strength 40mA instead of 25mA.
Memory interface, but less flexible and requires latches I wouldn't use, so not usable.