Renaldas Zioma Aims to Do What Zilog Won't: Keep the Vintage Z80 in Production with the FOSS Z80

Developer, researcher, and self-described "neuromorphic chip enthusiast" Renaldas Zioma has reacted to the discontinuation of the classic Zilog Z80 in a practical manner: by designing a fully-compatible open-hardware replacement, a prototype of which is due to be manufactured as part of the next Tiny Tapeout fabrication run.

"It is a time for open source and hardware preservation community to step in with a Free and Open Source Silicon (FOSS) replacement for Zilog Z80," Zioma explains of his project. "GOAL: To develop a drop-in Z80 replacement in eight-bit home computers such as ZX Spectrum. The first fabrication of FOSS Z80 is scheduled for June of 2024!"

Zilog surprised the vintage computing community earlier this month when it announced that it was to discontinue the standalone CPU variants of the eight-bit Z80 microprocessor after nearly fifty years in production. Launched as the company's first product in 1976 and designed by former Intel engineer and company co-founder Federico Faggin and early employee Masatoshi Shima, the chip became one of the most popular designs for microcomputers — going head-to-head with rival MOS Technology's 6502 and its successors.

While there is still one more production run scheduled, with a last-time-buy (LTB) date of 14 June 2024, Zioma's planning ahead with the FOSS Z80 project — creating an open source clone of the chip built using the open source SkyWater 130nm process design kit (PDK) and the OpenROAD workflow. While a 130nm process is far from the bleeding edge, where chipmakers have moved to manufacturing processes stated in single-digit-nanometer measurements, it's a lot more advanced than the Z80's original 4μm process node — over 30 times smaller, in fact.

While Z80 cores capable of running on a field-programmable gate array (FPGA) have existed for a while — and Zioma's version taking one of these, Guy Hutchison's TV80 core, as a base for the project — the FOSS Z80 is different in that it will be produced in silicon as a physical chip. This, Zioma explains, will be achieved using the Tiny Tapeout project, a multi-project chip platform that sees compact designs bundled together to dramatically reduce the cost of production — delivering a working chip for as little as $150, depending on the complexity of the design.

Zioma's design won't be a drop-in replacement for a DIP-packaged Z80 — at least, not at first. The Tiny Tapeout version, though, will serve as a test-bed to prove the design works — and will then be followed by a larger-scale production run through the Efabless chipIgnite platform to build an application-specific integrated circuit (ASIC) version for installation into a yet-to-be-designed adapter board offering a DIP40 pinout compatible with the original Z80.

Should the project get that far, Zioma has even proposed attempting to design a variant that would mimic the layout of the original Z80 die — originally created by laying out all 8,500 transistors by hand.

The source code for the project has been published to GitHub under the permissive Apache 2.0 license, while the version submitted to Tiny Tapeout is visible on the website — taking up a 2×2 tile block of the shuttle for Tiny Tapeout 7.