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Explaining the question more, I see some die pictures which are implementing a Cortex-M0, with Bluetooth LE and so on, depending on the chip functionality, and are appearing like this (nRF51822):

nRF51822

While on older CPUs I cannot see much digital "fuzzy" logic implementation, like this (AMD386):

AMD386

After googling a bit, it seems that today's ARM implementations are made with standard cells (creating the amorphous shapes on the die). So I can say that the "fuzzy" implementation on the first picture is the Cortex itself.

I understand that all the regular shapes may be memories and all the "hand-drawn" parts are analog. So I wonder, in the past were the analog designers to implement the digital parts under the guide of digital guys who were defining the architecture?

What am I missing?

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    \$\begingroup\$ Worth pointing out that it's difficult to compare these two dies for a range of reasons: process (AM386 was 800 nm, M0 is probably <100 nm), type (CPU vs micro - memories are very different), generation (1991 vs 2009 - vastly different tooling) etc. \$\endgroup\$ – awjlogan Sep 16 '18 at 13:37
  • \$\begingroup\$ To enforce your point, it was also difficult for me finding an equivalent modern die picture non in standard cells, if existing at all. \$\endgroup\$ – thexeno Sep 16 '18 at 22:08
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No, analog IC designers were not creating microprocessors under the direction of digital architects. It's more correct to say that digital integrated circuit designers needed to know quite a bit about how the transistors actually behaved.

The choice of whether a particular part of a digital IC will be crafted using standard cells or hand-drawn circuits is simply a matter of economics. For dense, highly-repetitive structures like memories (cache, microcode ROM, register file) it made sense to invest time in handcrafting the few basic cells that would be tiled together...the result was much smaller and faster than an equivalent circuit made from standard cells.

Blocks of "random logic" such as state machines and small counters were created using standard cells and CAD tools. Creating hand-crafted layouts for these blocks would have taken an enormous amount of time and provided little benefit. Instead, people worked to make the CAD tools and cell libraries better.

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The "fuzzy" appearance you're seeing in the first microphotograph is indeed the result of an automatic place-and-route algorithm synthesizing a design using a standard cell library.

Older parts, like the AMD386 in the second image, predate the existence of these algorithms. The entire design was painstakingly placed and routed by hand, accounting for its more orderly appearance. (On the other hand, the lack of a single process producing the layout accounts for the large areas of interconnects required on the 386 -- a PAR algorithm probably could have produced a much more efficient layout.)

Note that this doesn't mean that there's anything analog about the AMD386. It's an entirely digital part! Analog components in modern chip designs often look more "orderly" because they contain physically large parts which must be spaced apart from other logic, like the circular inductors in the lower right of the nRF51822.

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  • \$\begingroup\$ "AMD386. It's an entirely digital part! " - I would like to make a note about a thing: at least the IO pads and things like that (assuming no internal regulators, oscillators and the like), thay have to be analog. There are protection diodes, reverse protection diodes maybe, different transistors of the inverters for the fan-out/in. And who knows what I am missing. I am not sure it is even existing a pure digital IC in the worlds, made out only of standard cells and couple of VDD/GND lines. \$\endgroup\$ – thexeno Sep 27 '18 at 13:01

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