I’ve assembled more of the front panel, but still can’t get photos uploaded to flickr. I’ve ordered connectors and ribbon cable from Digikey, and paid for overnight shipping so they should be here on Tuesday.
I’ve written a model of a 256×8 memory to match the original Elf, and an “elf.vhdl” wrapper to integrate the 1802 core, memory, clock control, reset control, switch debouncing, LED output, etc. Contrary to my original plan, I have not tried to make it bus-compatible with the Elf. That isn’t really necessary, and time’s running short. I need to start on the VHDL testbench so I can do some verification using the simulator (GHDL). That’s far easier than debugging it on the FPGA itself.
I’m thinking about what demo programs to use. The sample programs in the original “Build the COSMAC Elf” article series are a bit more simplistic than I’d like. I’ll probably use an adjustable-speed counter program, to demonstrate programmatic use of the switches and LEDs. If there’s time, maybe I’ll add a piezo beeper and write a simple music program. The latter might not fit in 256 bytes, so I may have to set up a mode to allow the use of more memory. (The FPGA has enough BlockRAM to fill the entire 64KB address space.)
Things are really coming down to the wire!