Ken Shirriff explains in detail how to reverse engineer CMOS integrated circuit chips, illustrated with a vintage Soviet counter chip.

I recently came across an interesting die photo of a Soviet chip, probably designed in the 1970s. This article provides an introductory guide to reverse-engineering CMOS circuits, using this chip as an example. Although the chip looks like a tangle of lines at first, its large features and simple layout make it possible to understand its circuits.

I’ll first explain how to recognize the individual transistors. Groups of transistors are connected in standard patterns to form CMOS gates, multiplexers, flip-flops, and other circuits. Once these building blocks are understood, reverse-engineering the full chip becomes practical. The chip turned out to be a 4-bit CMOS counter, a copy of the Motorola MC14516B.

Read the full details on Ken’s blog post here.