Engineer Pepijn de Vos has released the source code and video demonstration for a five-chip 74-series logic gate ‘breathing LED circuit’ – without ever having had to manually design the printed circuit board.
“I had been keeping an eye on Yosys, the open source HDL synthesis tool, which can apparently do ASIC by giving it a liberty file that specifies the logic cells your foundry supports,” de Vos explains. “Meanwhile I also toyed with the idea of making a 7400 series computer, and I wondered if you could write a liberty file for 7400 chips. I had kind of dismissed the idea, but then ZirconiumX [Dan Ravensloft] came along and did it.
“It suffices to say this revived my interest in the idea and a lively discussion and many pull requests followed. First some small changes, then simulations to verify the synthesised result is still correct, and finally a KiCad netlist generator. You see, generating a Yosys netlist is nice, but eventually these 7400 chips have to end up on a PCB somehow. Normally you draw your schematic in Eeschema, generate a netlist, and import that to Pcbnew. But instead I used skidl to generate the netlist directly. Then all there is to do is add the inputs and outputs and run the autorouter (or do it manually of course).
“This was all done in Verilog, so where is the VHDL, you might wonder. Well, Yosys does not really support VHDL yet, but Tristan Gingold is hard at work making GHDL synthesise VHDL as a Yosys plugin. I think this is very important work, so I’ve been contributing there as well. After some pull requests I was able to port the breathing LED to VHDL.”
De Vos’ full write-up, and linkes to the source code, can be found on his blog, while the resulting circuit is demonstrated in the embedded video.