Flex-ATX is the smallest industry-standard form factor specification, and it defines a board that is up to 9''x7.5'' in size. Note the up to part of the dimensions, which means that, even though those dimensions are the maximums, less is also allowed.

Therefore, a flex-ATX board can be smaller than that, but how much smaller? By analyzing the flex-ATX specification—and, in particular, studying the required mounting screw locations—you can see that a flex-ATX board could be made small enough to use only four mounting holes (C, F, H, and J).

According to the flex-ATX standard, the distance between holes H and J is 6.2'', and the distance between hole J and the right edge of the board is 0.25''. By leaving the same margin from hole H to the left edge, you could make a board with a minimum width of 6.7'' (0.25'' + 6.2'' + 0.25'') that would conform to the flex-ATX specification.

Similarly, the distance between holes C and H is 6.1'', and the distance between hole C and the back edge of the board is 0.4''. By leaving a minimum 0.2'' margin from hole H to the front edge, you could make a board with a minimum depth of 6.7'' (0.4'' + 6.1'' + 0.2'') that would conform to the flex-ATX specification.

By combining the minimum width and depth, you can see that the minimum board size that would conform to the flex-ATX specification is 6.7''x6.7'' (170mmx170mm). VIA Technologies Platform Solutions Division wanted to create a motherboard as small as possible, yet not define a completely new and incompatible form factor.

To accomplish this, in March 2001 VIA created a board that was slightly narrower in width (8.5'' instead of 9'') but still the same depth as flex-ATX, resulting in a board that was 6% smaller and yet still conformed to the flex-ATX specification.

VIA called this ITX but then realized that the size savings were simply too small to justify developing it further, so it was discontinued before any products were released. In April 2002, VIA created an even smaller board that featured the absolute minimum width and depth dimensions allowed by flex-ATX. It called it mini-ITX.

In essence, all mini-ATX boards are simply flex-ATX boards that are limited to the minimum allowable dimensions. All other aspects, including the I/O aperture size and location, screw hole locations, and power supply connections, are pure flex-ATX. A mini-ITX board fits in any chassis that accepts a flex-ATX board.

However, larger boards will not fit into a mini-ITX chassis. The mini-ITX form factor was designed by VIA especially to support VIA's low-power embedded Eden and C3 E-Series processors. Only a very small number of motherboards is available in this form factor, and only from VIA and one or two other manufacturers.

Because the processors used on these boards are substantially less powerful than even the Intel Celeron 4 or AMD Duron entry-level processors, the mini-ITX form factor is intended for use mainly in nontraditional settings such as set-top boxes and computing appliances. The size of the ITX and mini-ITX boards relate to flex-ATX.

Again, I must point out that technically any ITX or mini-ITX board conforms to the flex-ATX specification. In particular, the mini-ITX is the smallest board that can conform.

Although the still-born ITX format was virtually the same as flex-ATX in size (which is probably why it was discontinued before any were sold), mini-ITX motherboards are 170mmx170mm (6.7''x6.7''), which is 34% smaller than the maximum allowed by flex-ATX.

To take advantage of the smaller mini-ITX format, several chassis makers are producing very small chassis to fit these boards. Most are the shape of a small cube, with one floppy and one optical drive bay visible from the front. The layout of a typical mini-ITX motherboard, the VIA EPIA-V, is shown in Figure below.

As Figure above makes clear, mini-ITX motherboards can offer a full range of input-output ports. However, several differences exist between the mini-ITX motherboards and other ATX designs:

• The processor on a mini-ITX motherboard is usually permanently soldered to the board, making future processor upgrades or replacements impossible.

• Most mini-ITX chassis use TFX power supplies, for which there are currently only a few suppliers. Consequently, replacements for them are more expensive and more difficult to find.

• The available TFX power supplies are rated for less output than larger supplies, typically up to 240 watts maximum.

• There is no provision for replacing onboard video with an AGP video card.

Because mini-ITX boards and chassis are made by only a small number of suppliers, future upgrades or parts replacements are limited. However, mini-ITX boards are actually flex-ATX boards, so they can be installed in any standard flex-ATX, micro-ATX, or full-size ATX chassis and use the corresponding power supplies.

The only caveats are that the smaller mini-ITX chassis will not accept larger flex-ATX, micro-ATX, or full-size ATX boards and most mini-ITX chassis accept only TFX power supplies.

When you select a mini-ITX system, you must be sure to select the appropriate processor type and speed necessary for the task you need it to perform because processor replacements or upgrades almost always require changing the entire motherboard.

Both the VIA C3 E-series and the VIA Eden are x86-compatible processors, so they run the same operating systems and applications as typical AMD and Intel processors, including Windows and Linux. However, the C3 and Eden processors are significantly slower than the Celeron 4, Pentium 4, and Athlon XP processors found in typical desktop and notebook computers.

The Eden ESP series of processors uses a simplified design optimized for the most common operations. Eden ESP processors are an excellent choice for set-top boxes and Internet clients, but they are not as powerful in features or clock speed as the C3 E-series.

VIA recommends Eden processors for embedded systems because the processor can be operated without a fan by using a passive heatsink for cooling. For more traditional computer tasks in a small footprint, VIA recommends the C3 E-series because it has performance similar to Intel Celeron processors running at similar speeds.

C3 E-series processors can also be run with a passive heatsink, but they run hotter than Eden-series processors because they use higher voltages. VIA recommends a cooling fan for C3 E-series processors unless the case is specially designed to provide adequate cooling for the processor.

VIA uses varying combinations of the following North Bridge and South Bridge chips in its mini-ITX motherboards. The North Bridge uses either the PLE133 or CLE266 chips, whereas the South Bridge uses either the VT8231 or VT8235 chips.

The PLE133 North Bridge chip features built-in Trident AGP 4x video and support for PC100 and PC133 SDRAM memory. The CLE266, on the other hand, features built-in S3 Savage 4 4x AGP video, a built-in MPEG2 decoder for excellent DVD playback, and support for DDR266 SDRAM memory.

The VT8231 South Bridge chip features AC'97 audio, MC'97 modem, an ATA-100 host adapter, and four USB 1.1 ports. Additional capabilities can be added through optional chips. The VT8235 South Bridge chip features six-channel audio, an ATA-133 host adapter, USB 2.0 ports, 10/100 Ethernet, PCI controller, and MC'97 modem.

It also supports the new 8X V-Link interface with the North Bridge. VIA offers a handful of mini-ITX motherboards through its VIA Platform Solutions Division (VPSD), including the following:

• EPIA

• EPIA V

• EPIA M

• EPIA M10000

The EPIA M motherboards are the most sophisticated of the three and are the best choice for digital multimedia because of their faster memory subsystem, USB 2.0 and IEEE-1394a ports, and optimized DVD playback. Obviously, with top performance only on par with sub-1GHz Celeron systems, mini-ITX motherboards are not intended for power user applications.

However, if you need a compact system for specialized uses such as home entertainment centers or for small-footprint computers for office suites and Internet access and don't mind investing in a small form factor that might make future upgrades or repairs extremely difficult, these tiny systems can be useful.