It is legitimately bugging me that I seem to be the only person who wants to be able to buy an ARM or RISC-V or MIPS or SPARC CPU, implemented with an inexpensive FPGA, with a standard parallel memory bus.
I feel stupid for wanting to be able to take such a thing, stick two or four 8/16 bit wide SRAMS and ROMs on it, and have a play like it was Real Hardware™.
@funkylab Let's say it's a hypothetical RISC-V variant, which is 32-bit internally, but has a 16-bit data bus (like the old 68K CPUs) and a 24-bit external address bus (32 internally, top bits always 0). That's 40 pins right there. You'd also need:
/IRQ
/NMI
/RES
CLK (input or output, whichever is more convenient)
R/W or a /RD and /WR pair
So let's call it 46 I/O pins needed, and a RISC core that knew how to handle spreading a 32-bit read or write over 2 cycles.
@funkylab If it's on a small PCB with components only on the top side, it can have Pi Pico style castellated holes for board mounting. Make it a square-ish board, with a notch on a corner to signify pin 1. Say you need a power and ground on each side, so that's 13 castellated holes per side, 2.54mm pitch, that's ~30mm per side, call it 35mm square just to be safe.
Use a BGA package FPGA, give it the regulator and capacitor support bits it needs on the module, so it can talk 3.3V bus.
@mos_8502 yeah, the problem really will be spec'ing this for low price; can't do it with less than ca 48 GPIOs , and there's simply cost to having enough silicon die to connect that many pads/balls/pins, so the cheapest FPGA families probably won't do. and then we're pretty quickly in >11€/piece territory for the FPGA… add in 1€ for config memory, 1€ for power, 1€ for passives, plus PCB, plus assembly.
Honestly, I could see me spinning a cheap 4-layer board for such an endeavor, but even if …
@mos_8502 upfront R&D, prototype costs,
then your price would have to be (taxes+per piece fees)+(R&D cost)/1000 above per-piece costs.
costs: 11€ per FPGA, (cheapest I can find on LSCS,), 4€ other components, 5€ board and assembly costs = 19 € per piece
Say you need another 45s per piece for programming, a minimal functional test, and putting a sticker saying "MIPS" on it, and you do that for 30 €/hr (smwhat minimum cost for labor from POV of an employer), incl sticker you're down +50ct;
Cmod A7 The Cmod A7-15T variant is now retired and no longer for sale in our store. The Cmod A7-35T is still available. The Digilent Cmod A7 is a small, 48-pin DIP form factor board built around a Xilinx Artix 7 FPGA. The board also includes a USB-JTAG programming circuit, USB-UART bridge, clock source, Pmod host connector, SRAM, Quad SPI Flash, and basic I/O devices. These components make it a formidable, albeit compact, platform for digital logic circuits and Microblaze embedded softcore p…
Studio 8502 
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