A home computer design puts its processor across 20 programmable logic chips

Justin Davis wants a home computer whose parts have legs you can actually reach with a soldering iron. His PLACARD design uses twenty ATF22V10C programmable logic devices and thirteen ordinary logic and memory chips. The processor is a collection of cooperating parts, with no single microprocessor taking the credit (or the whole socket).

Davis’s project page makes the manufacturing constraint unusually clear: every part in the intended machine is a through-hole DIP still in production. That avoids turning the parts hunt into an archaeological expedition. The bill of materials should lead to current components, even while the finished board aims for a much older way of building.

Programmable logic devices let Davis specify small collections of logic functions inside separate chips. The design distributes the arithmetic unit, instruction handling and address selection across them. The CPU has eight-bit data and a sixteen-bit address bus. Instructions take two, three or four clocks, so quoting the clock frequency alone would oversell how much work gets done in a second. Even a homemade computer deserves protection from benchmark brochure math.

The selected clock is 25.175 MHz, which is also the VGA pixel clock. Video is part of the same design exercise, rather than a modern graphics card quietly doing the difficult bit off camera. Davis specifies 640 by 480 VGA with up to 256 colors, alongside eight-bit sound. PS/2 keyboard and mouse connections and two SNES controller ports supply a suitably crowded set of ways to interact with it.

The current address is a Basys 3 FPGA development board. An FPGA lets Davis test the logic before manufacturing the intended board of separate chips. He reports that everything operates on that prototype and that software development can continue there. It is a useful intermediate result, with a large asterisk attached to any photograph someone might mistake for the final machine.

The page says the KiCad schematics are finished, but the main board is “95% routed.” That leaves real work before ordering a board. Calling the design fab-ready would erase the very last step the maker says he is still doing. Ninety-five percent has a habit of being the longest five percent in the room.

The source’s expected next milestone is a manufactured board that matches the prototype’s behavior. Keeping those two stages separate makes later progress more interesting: a working FPGA already answers whether the design can execute, while the physical build must answer whether this particular collection of chips and traces behaves as intended. Solder has a way of asking follow-up questions.

The empty sockets still get the final say.