What does "system on a chip" really mean anyway?

[Charles Kozierok]

I've been wondering about the use of this term for quite some time. It obviously has great marketing appeal, implying greater integration than merely calling a chip a "CPU". But it seems that nobody really agrees on what it means, and its definition is both muddled and evolving.

The term "north bridge" was the previous one to be used and abused in this manner. I think people started referring to the CPU having an "integrated northbridge" way back in the K8 days.. even though those CPUs generally did not include all of the capabilities generally considered part of the north bridge. Modern chips do, and it's reasonable to say they have an integrated (what used to be) "north bridge".

Now it's "system on a chip", and I see little agreement as to what this really means. I believe some people started using this term as early as the first chips with integrated graphics. But while one could argue that the CPU and the GPU are the heart of a system, they aren't the entire system by any means.

Obviously no SoC is really going to have everything in it, and it's not reasonable to scoff at the term unless it includes every bit and piece. After thinking about it a bit, I decided that the "line in the sand" for me would be whether or not the CPU still connects to a chipset. I've always considered the chipset to be "the system", and so IMO if a chipset is still needed, I can't consider a CPU to be a SoC, even if it has a lot of goodies integrated onto it.

Unfortunately, even this definition is falling apart these days. To wit: Intel's new Atom "Centerton" server chips. They call these chips "SoCs", and they are not designed to connect to a chipset. However, they do not include many of the standard interfaces that are now considered standard on a system, such as USB, SATA or Ethernet. These are essentially pushed off onto the motherboard makers, who have to use third party chips.

This almost seems to me like a case of de-integration, because multiple devices may now be required to do what used to be provided by the south bridge.

This use of "SoC" is especially dubious IMO for a product that Intel is hyping based on how little power it consumes. It's handy to be able to say you have a server chip that takes only 6W and requires no chipset, and just say "pay no attention to those other chips behind the curtain".

What do you think?

 

[Idontcare]

I like your example of the Atom, it drives home for me what the primary difference is. And that is in order for a chip to be dubbed a SoC the platform itself must be inspected.

Is the Atom chip a SoC if the platform (the system in question) requires secondary chips to make the system fully-featured?

If the answer is "additional chips are needed" then the CPU in question is clearly not a SoC when it comes to its usage in that specific platform.

A system on a chip or system on chip (SoC or SOC) is an integrated circuit (IC) that integrates all components of a computer or other electronic system into a single chip.

http://en.wikipedia.org/wiki/System_on_a_chip

For its intended purpose, I don't believe there has ever been an Atom-based product for which the Atom actually plays the role of a SoC.

I could be wrong there, but I don't recall a single example in which Atom is used without relying on secondary chips to make the system function as intended/designed.

 

[dave_the_nerd]

Not being too familiar with most non-x8 stuff, are there any SoCs that clearly meet those criteria (everything, from the CPU/GPU all the way down to the USB controller, all on one chip), or is the SoC still an academic concept that represents the endgame of the integration trend?

 

[Ken g6]

Wikipedia seems to agree with you:

In short, for larger systems, the term system on a chip is hyperbole, indicating technical direction more than reality: increasing chip integration to reduce manufacturing costs and to enable smaller systems.

I would also say, based on that article, that most of the new CPUs called SoCs aren't because they don't integrate one key component: RAM. I would also say that's generally a good thing, so that RAM can be upgradable.

For server systems, it makes sense to me not to integrate many of the interfaces as well. Some systems may act as MPI nodes, requiring Ethernet but not USB or SATA. Others may be integrated controllers that require USB and SATA, but should not have Ethernet available to prevent malware and hacking.

 

[Charles Kozierok]

Ironically, one could make the argument that Cyrix's MediaGX chip was more of a "SoC" than many chips touted as such these days. It had a CPU core, graphics engine, display driver, memory controller and PCI bus controller, all on one IC. It still used a south bridge for ISA (and other legacy stuff), but otherwise IIRC it was all there.

Of course it was all primitive stuff, but then we're talking 1997....

This is all going to get much more attention with Broadwell, I'd expect, when they put the CPU and south bridge in a single package.

ETA: Just noticed the Wikipedia link talking about the AMD Geode, which evolved from the MediaGX.

 

[Blitzvogel]

For me, SoC seems to coalesce with CPU + GPU + some form of memory (these days limited to just relatively small caches) all photolithographed onto a single substrate.

 

[Charles Kozierok]

I think it's going to be a long time before we see logic and DRAM on the same substrate... they just have requirements sufficiently different that it may not make sense.

What we'll see instead is the sort of stacked packaging Intel used for Medfield and Clover Trail ... which are the only two chips they've produced so far that I feel comfortable actually calling "SoC".

Incidentally, it seems some ARM folks aren't too impressed with Centerton either.

 

[lopri]

Generally speaking: CPU + GPU + Memory controller + Modem on a single chip. This combination used to make up Intel's "Centrino" platform in the past with corresponding (multiple) chips.

 

[Charles Kozierok]

Seems like a pretty narrow definition, and one that would disqualify many of Intel's own claimed SoC products.

 

[lopri]

It is narrow and wide at the same time. It is a loose definition depending on what popular devices need to function in today's environment. On some devices, certain component may be considered less important or even unnecessary. Most consider, for example, Tegra 3 an SoC despite its lack of communication module.

But at the same time, I could see the chip makers including even more features as well as super fast RAM on it in the future on their SoCs. (considering that memory technology has been lagging behind processors for like ever)

SoC as a definition is a moving target.

 

[sefsefsefsef]

I like a definition that says "everything to make a computer work, except main memory, storage, and the actual I/O devices (monitor, keyboard, speakers, mouse)." In other words, pretty much everything that used to be part of the Northbridge and Southbridge 10 years ago, plus a CPU and GPU (plus maybe sound?).