What Is a System? It's Turtles All the Way Down...or Fleas

According to Steven Hawking, in his book A Brief History of Time, Bertrand Russell once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant turtle." The scientist gave a superior smile before replying, "What is the turtle standing on?" "You’re very clever, young man," said the old lady. "But it’s turtles all the way down!"

Electronic Systems

Electronic systems are a bit like that. What is a system depends on who you talk to, and a system to one person is built out of components that are themselves systems to someone else. Everything is really a system of systems.

In the EDA and semiconductor world, we are used to talking about systems-on-chip or SoCs. But the reality is that almost no consumer product consists only of a chip. The closest are probably those remote sensing transport fare-cards like Clipper that we now have in the Bay Area (finally, well over 20 years after Hong Kong’s Octopus card which was probably the first). They are self-contained and don’t even need a battery since they are powered by induction from the reader. Even a musical birthday card requires a battery and a speaker along with the chip to make a complete system.

SoCs

When we talk about system-level in the context of chip design, we need to be humble and realize that the chip goes into something larger, which some other person considers to be the system. Someone once memorably said to me that "system-level is one level up from whatever level you work at". I say "memorably" but the person I remember saying it denies saying it when I asked him if it was original. My memory got rowhammered, I guess.

People at the higher level often have very little interest in how the lower-level components were created. The RTL designer doesn’t care much about how the standard-cell library was characterized, the software engineer doesn’t care much about the language used for the RTL. People only care about their own turtle.

At each level, some model of the system is required. It seems to be a rule of modeling that it is very difficult to improve (automatically) the performance of a model by much more than a factor of 10 or 20 by throwing away detail. Obviously, you can’t do software development on an RTL model of the microprocessor; too slow by far. Less obviously, you can’t create a model on which you can develop software simply by taking the RTL model and reducing its detail and speeding it up. At the next level down, the RTL model itself is not something that can be created simply by crunching the gate-level netlist, which in turn is very different from the circuit simulation model. The process-development people (technology development or TD in semiconductor-speak) model implants and impurities in semiconductors but those models are not much use for analog designers; they contain too much of the wrong type of detail, making them too slow.

Two Ericsson Stories

When I worked at Virtutech (now part of Intel), Ericsson was a customer and they used Virtutech’s products to model mobile base stations, which is what the engineers we interfaced with considered a system. A base station was a cabinet-sized box that could contain anything from a dozen up to 60 or so large circuit boards, in total perhaps 800 processors all running their own code. Each base station is actually a unique configuration of boards so each had to be modeled to make sure that that collection of boards operated correctly, which was easiest to do with simulation. Finding all the right boards and cables would take at least a couple of weeks. But it is extremely important that the base station cannot be configured in some catastrophic way whereby it cannot be rebooted. If it does, the CEO of Ericsson gets the dreaded 2:00am phone call.

I was at a mobile conference in the mid-1990s where I talked to a person in a different part of Ericsson. Ericsson, at the time, had a huge business building out 3G networks all over the world. He did system modeling of some sort to make sure that the correct capacity was in place. To him, a system wasn’t a chip, wasn’t even a base station. It was the complete network of base stations along with the millions of cellphones that would be in communication with them. He thought on a completely different scale to most of us. His major issues were all at the basic flow levels. As it happens, there is a Cadence connection here. Twenty years ago, the Alta Group of Cadence had a product called BONeS (block oriented network simulator) for just this sort of modeling. This type of modeling is more like fluid dynamics than anything electronic. That turtle is many levels above mine. It's turtles all the way up.

My turtle is mostly the chip level. And most chips are built using an IP-based methodology. Some of that IP is complex enough to call a system in its own right. For example, a high-speed SerDes contains a DSP to do the equalization. And that DSP is built out of standard cells and memories. So it’s pretty much turtles all the way down, too.

A Weird Connection with Boolean Algebra

You have may have heard of De Morgan's laws if you ever studied Boolean algebra, which you probably did if you are reading this post, since it is the basis for digital electronics. In his 1872 poem titled "Siphonaptera", Augustus de Morgan wrote:

Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum.

It's fleas all the way up.

And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on.

And, just like turtles, all the way down too.

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