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Intel's BTX Form Factor - An Analysis and Opinion

Date: 2005-3-7

   ThoughtsAdvertismentThoughts I can count the number of commercially available BTX mainboards and chassis' in this country on one hand. I can count the number of UK retailers selling package...

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Introduction to BTX

Intels BTX Form Factor - An Analysis and Opinion
Intel's BTX form factor. Re-read that for a second and absorb its full meaning. Intel's BTX form factor. See what I'm getting at? If you visit Intel's current BTX website, tied as it is to their Pentium 4 processor, you'll see the following: "Intel has collaborated with the desktop computing industry to create an evolutionary step in the desktop computer form factor". I'm 100% certain that's true, don't get me wrong; I've been part of correspondence with the companies Intel has collaborated with to define BTX. It's the level of collaboration that bothers me. I'll return to that thought, so keep it in mind.

Intel's BTX Form Factor

The current ATX form factor, what you'll undoubtedly be using in your own home computer as I type this, has been around for nearly eleven years if I'm doing my sums correctly. Created back in the days of the frugal 486 processor (less than 1W TDP), graphics processors with less than a half a million transistors, and hard drives that spun at less than 5400rpm, hot wasn't a word you would use to classify PCs of ten years ago.

Fast forward to December 2004 when 100W TDP processors are on the open market, SLI exists to let you put two of NVIDIA's toasty 6800 Ultra boards in a computer system, people regularly have more than one 7200rpm (or 10000rpm, or 15000rpm) hard disk and power supplies to get it all going run into the 600W area now and create serious heat (which isn't helped by people requiring them to be silent with one or fewer fans!) and you can see that the ATX form factor that we're using to keep everything in check is beginning to show signs of age, in terms of keeping thermals in check.

It's not just thermals that need to be considered either. Size is driving forward a new wave of computing designs that throw ATX out of the window in the quest to make the modern PC ever smaller. So ATX is pushing against two things that are fighting hard against it. There's the microATX subset of full ATX which chops the bottom of an ATX mainboard off (if you'll allow me to be general), but it only tackles one of the fronts that seek to kick it while it's seemingly down.

In steps BTX, a reshuffle of the innards of your PC and a somewhat strict method of laying down that reorganisation of your PC's internals. So what does BTX seek to do? You can argue that BTX is nothing more than ATX turned upside down, to get your CPU at the bottom of your case in line with intake fans that already pull in air across that part of a chassis, but to say so would be to miss the point.

BTX does move the CPU area of a mainboard somewhere else in a chassis, but it moves the rest of it round too. Think of your current PC's layout as a series of layers and imagine the following.

Your regular ATX PC, and I'm talking a tower-like chassis here since that's the system that BTX was created to help out the most, is a bit like this:

PSU -- CPU -- E -- DISKS
X----- NB --- M --- DISKS
-------- SB ------ DISKSX
------------------- DISKSX

It's a bit like that, right? Your PSU sits somewhere near the CPU and often you'll use that PSU as an exhaust for CPU heat, being as it is at the top of your case. Past that you'll have your memory modules oriented north-south up and down the mainboard. The northbridge sits under the CPU as it has to (I'll ignore single ASIC core logic here, although the principles are the same) and your graphics card sits on the far side of that bridge.

The southbridge I/O ASIC sits in the right hand corner of the bottom of the mainboard feeding your PCI slots and powering the audio, disk controller and whatnot of a regular PC. Then your disk drives sit on the right, opticals at the top and hard disks from the middle to the bottom in any number of configurations.

The X characters in my super-detailed diagram represent fans in the average desktop case, pulling in air from the front bottom and spitting it out at the rear, somewhere near the top via an exhaust fan and the CPU. Picture that airflow for a second. From the bottom of the case, over some disk drives usually, where it's magically supposed to flow over a graphics card that has its own fan moving heat off of itself, across the northbridge area and out of the exhaust fans without ever really hitting the CPU area, which you want a powerful, silent PSU to cool instead. I'm not wide of the mark when I describe a massive number of PC chassis' like that. Does it make much sense to you either?

Of course, you have exceptions to the rule. Chassis' conceived with thermals in mind are out there and while they might be fugly on occasion, for me that's a small price to pay in return for a chassis that'll keep a high-powered PC from impersonating Chernobyl. Over the top, certainly (and definitely in poor taste) but you can hopefully see my point. ATX doesn't really work for the modern PC. Extended ATX, the even larger variant that accomodates server and 'workstation' mainboards in ATX-esque chassis is a second curiousity to consider. More on that later.

So BTX wants to change everything and sort the entire form factor out. But how?

Thermals - Wind Tunnels!

Well, not quite, but close. BTX picks up the major components in your PC system and puts them down in different places. The reorganisation of components creates thermal compartments where air flows from the front of the case to the back (right to left in our current way of thinking), across a set of components that resides in that layer. So you've got it set out like this, from top to bottom in a theoretical BTX tower chassis.

PCI and/or PCI Express IO
AGP and/or PCI Express Graphics
Southbridge, Northbridge and CPU
Memory modules
PSU and Disks

Notice I've highlighted the CPU layer that has the core logic inline? The northbridge is still in the middle of the board, allowing easy routing of traces from it to and from the multi-lane PEG or AGP I/O above and the memory slots below. That applies to potential Athlon 64 BTX systems with the trace routing from CPU socket area to memory modules. The CPU layer is the only one defined in the BTX specification that requires active airflow across the layer, firstly over the CPU and then out of the rear of the case after passing across the core logic.

The current specification only seems to define microBTX and picoBTX, rather than a full BTX or Extended BTX specification that I'm theoretically discussing right now, and one would assume that such specs would include the requirement for active airflow across the graphics layer in the case, from front to back.

Notice that the PSU moves from the top of the case to the bottom, sharing a thermal layer with any disk drives you have installed in your case. Optical drives in a desktop case would share the same thermal layer as the topmost I/O section, assumedly. The microBTX spec and current supporting chassis' define optical drives sharing that disk layer too.

So the case is sectioned, layered like a cake, from top to bottom, with air flowing across those layers from right to left (front of the case to the rear) while maintaining sensible component placement for trace routing, putting hot components on individual layers and in general, forcing a logical layout to the mainboard and thus that which finds itself contained in a BTX chassis.

Electrical Simplification

Electrically, in terms of necessary component placement, trace routing and signal integrity, ATX mainboards are a mess. Cramped designs, the necessity that the voltage regulator circuitry is all on the top side of the board, hell, the necessity that all major components reside on the top of the board and other such factors all conspire to make it an electrical nightmare to layout the majority of ATX mainboard designs.

The separation of BTX into layers was also made with the electrical concerns of a mainboard design in mind. Notice how it's sort of winged, the core logic and processor in a strip and the expansion I/O and memory modules lie either side in their layers? That enables BTX to be simpler in terms of electrical design. The voltage regulator circuitry can be moved away from the CPU bus traces. The memory traces aren't sharing the same board space as expansion I/O traces. Everything gets room to breathe, resulting in cleaner signals and simpler routing (which makes the board cheaper).


The big factor for any component maker, or consumer of technology, is cost. If it's expensive, it won't find sales. Intel argue that BTX's simpler mainboards enable them to be cheaper, and I largely agree. Volume will play a part in that and it's volume that's the main push towards low-cost components and full systems, but the spec helps to achieve that for the most part.

What about BTX for larger mainboards?

It exists, but the push is towards the smaller form factors at the moment. Browse and you'll see BTX for larger tower chassis designs. Put simply, BTX for larger mainboards and chassis' are the picoBTX core with extra I/O slots for PCI Express and PCI Conventional. There's no real set spec for dual PEG in BTX, but the PCI-SIG are working on it. So if a vendor wants to create a mainboard that would fit in the same space as an Extended ATX mainboard would today, they extend a picoBTX design downwards. Intel claim that makes it simpler for mainboard designs to be conceived and it'll reduce time to market.

So how are we supposed to migrate to BTX if there's no chassis' to support it and boards we can use?

You're not. It's nowhere on any BTX website, nor in any publically available communications about BTX that Intel have published, but talking to Intel about BTX outside of the usual press channels had them confirm that the rollout timetable for BTX is in the order of years. It's something quite important to consider. Intel don't expect ATX to die out any time soon, if at all. It's a slow, measured adoption of BTX that they're looking for, in markets that require it first, then everywhere else that it makes sense to deploy BTX into. ATX, or at least refined ATX, may very well be what we use for the PC until it ceases to exist as we know it. At least Intel are happy for that to occur.

What they want to do - which is not what they seem to convey as a company and maybe is something for them to change outwardly - is quietly work with whoever wants to create a BTX design and help them do so. If a vendor isn't interested, Intel aren't interested in strong-arming them into a market they're not ready for. It's the tortoise versus the hare with BTX. Slow and steady adoption wins the race for them, not massive at-once platform shifting.

Quite the opposite of what usually happens at Intel (outwardly at least), the company wishing to mould BTX into a market for the platform, rather than creating one by force.

That means a certain amount of marginalisation for BTX in the beginning. But that's by choice, and it's something I tried to put across to Intel in previous communication but which is easier to explain after talking to them about timetables for BTX. Confining BTX to only the markets that request it is key to its success and Intel confirm they're working to that end.

   [1]· Introduction to BTX
   [2]· BTX in the flesh
   [3]· Noise and PSUs
   [4]· Thoughts
   Next Page

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