BIOS and tweaking software
Despite a smaller system stature, Shuttle's XPC BIOSes have always been loaded with tweaking and overclocking options. The SN21G5's BIOS is no exception.

Most of the SN21G5's tweaking options are consolidated under Advanced Chipset Options. Here, you can play with the HyperTransport connections between the north and south bridge chips, and between the north bridge and processor. A couple of overclocking options also creep into the mix, with the BIOS allowing users to set the HT clock between 200 and 300MHz and the PCI Express clock between 100 and 145MHz.

The remainder of the SN21G5's overclocking options are tucked away in the frequency and voltage control section of the BIOS. Here, you can manipulate the processor multiplier and the chipset, memory, and processor voltages. CPU voltages go all the way up to 1.7V, which is pretty impressive considering the fact that many full-size ATX boards top out at around 1.55V. Small form factor systems tend not to be particularly good overclockers due to their limited air flow, though.

To get at the SN21G5's memory timings, we have to go back to the Advanced Chipset Options menu. All the important memory settings are there, although the BIOS doesn't provide access to some of the more seldom-used timings available on other boards. The BIOS also doesn't allow for an asynchronous memory clock faster than 200MHz, either.

Shuttle's XPC systems had temperature-based fan speed control way before it was cool, and the SN21G5 doesn't disappoint. Users can choose from nearly a dozen fan speed profiles, each of which offers a different balance of lower noise levels and better cooling performance. Shuttle offers even more control over fan speeds through its XPC Tools Windows software. However, Shuttle doesn't currently have an XPC Tools config file that works with the SN21G5, so it's impossible to run the app on the system.

Speaking of software that doesn't work with the system, NVIDIA's excellent nTune system utility doesn't recognize the SN21G5 as an nForce platform. Shuttle's working on the problem, and expects to support nTune in the future, but it's not ready just yet.

Since the SN21G5 isn't due to be available for a few weeks, it's hard to get too worked up about missing XPC Tools and nTune support. However, it's a little surprising that Shuttle is releasing yet another XPC system without BIOS-level alarm or shutdown conditions. The SN21G5 relies on a single fan to cool the processor and chassis, and if that fan fails, rising temperatures could damage system components. To protect against damage due to a catastrophic cooling failure, the SN21G5's BIOS should at the very least include a fan failure or temperature-based system shutdown trigger.