Vintage computing enthusiast and hardware engineer Reinhard "c0pperdragon" Grafl has put together a MOS 6502-based computer with a difference: the entire thing fits on to a compact breadboard and uses only four integrated circuits plus a crystal oscillator.
"Designs for SBCs [Single-Board Computers] based on the [MOS Technology] 6502 are quite common, but normally they use some specialized ICs [Integrated Circuits] to handle the IO [Input/Output]. I did some designs myself using standard 74-logic for the auxiliary functionality," Grafl explains. "Pushing the minimalist approach to the extreme, I decided to find a way to fit a 65C02-based computer into five ICs including oscillator, CPU, RAM and ROM."
Before setting about building the surprisingly compact machine, Grafl set himself four key specifications: it had to be able to run a WDC 65C02, a modern incarnation of the classic MOS 6502, at 10MHz; it had to include 32kB of RAM; it had to offer 32kB of ROM storage; and it needed to use software-driven IO, to reduce the component count. Oh, and it needed to fit into a single BB630 breadboard.
"The BB630 board is big enough to fit the mandatory CPU/RAM/flash ICs and one additional small IC," Grafl explains. "This additional IC has to somehow implement all he necessary logic for both the memory addressing as well as for the IO. A quad two-input NAND (74HC00) is a pretty versatile option here. Some very narrow spaces between these ICs can be used for a bit of analog circuitry."
For those looking to build their own Grafl has uploaded KiCad project files for the SBC, including a schematic and a proposed PCB design, to GitHub under an unspecified open source license, along with a brief demonstration application. "I don't indent to do much more programming on this device," Grafl admits.