AMD's SB800-series fan speed controller exposed


Options aplenty make this motherboard reviewer swoon
— 11:00 AM on October 26, 2010

As TR's resident motherboard reviewer, I've poked around in a lot of BIOSes over the past decade or so. They've come a long way in that time, adding integrated flashing utilities, support for multiple configuration profiles, and, of course, the ability to tweak a staggering array of memory timings, clock speeds, and voltages. On the overclocking front, we've long passed the point of diminishing returns. Any halfway-decent motherboard BIOS has more than enough clock and voltage ranges for enthusiasts, and most go above and beyond what even truly extreme liquid-nitrogen-fueled overclockers might require.

PC enthusiasts have always had a bit of an obsession with maximizing performance, so it's no surprise motherboard makers have lavished us with a wealth of overclocking and performance tweaking options. Few seem to have picked up on the fact that we also want our rigs to be as quiet as possible, though. The fan speed controls available in most modern BIOSes are basic at best and often laughably inadequate when compared to the frankly excessive overclocking options being offered.

This argument might sound familiar because I've been grinding this axe for a while now. I went off on a bit of a rant on the subject more than a year and a half ago, and little has changed since. Of the big three, only Asus has made real progress in improving BIOS-level fan speed controls, and it still has a long way to go to match what now-defunct mobo maker Abit was doing more than seven years ago. MSI's fan speed controls remain limited but continue to offer at least some tuning options, while Gigabyte's are still shockingly nonexistent. So, imagine my delight when, at AMD's Technical Forum in Taipei last week, I came across a presentation on the fan speed controls enabled by the company's current generation of south-bridge chips.

Motherboard manufacturers typically rely on SuperIO chips or other auxiliary silicon to adjust fan speeds based on system temperatures. Putting this functionality in the chipset certainly makes sense, but has AMD implemented the sort of functionality enthusiasts might want?

Oh yes—and in spades.

The SB800 family of south-bridge silicon has an integrated microcontroller that, with a little help from firmware built into a system's BIOS, can offer more control over that system's fans than has ever graced a motherboard. The microcontroller can manage fan speeds using three algorithms: step, linear, and non-linear. In step mode, fan speeds jump from one value to the next as temperatures rise. The linear algorithm smooths things out, allowing fan speeds to ramp, well, linearly between predefined minimum and maximum values. Combine the two, and you have a non-linear approach that's really more multi-linear. This method drapes a string of linear slopes across multiple step points.


I couldn't not graph them. This is TR.

Linear mode is most akin to what's available in contemporary BIOSes, although users aren't always able to set minimum and maximum temperature thresholds and fan speeds. Step mode is a little like the old-school high/low fan options that used to be prevalent before linear controls emerged. Instead of being limited to single high and low values, AMD's step mode can accommodate no fewer than eight predefined fan speeds and corresponding temperatures. Non-linear mode is just as flexible, allowing users to configure as many slopes as they can steps.

There's plenty of granularity, too. Fan speeds can be set in 1% increments, and temperature triggers can be adjusted 1°C at a time, giving users plenty of opportunity to perfect the effective shapes of their fan speed curves.


Single- vs. dual-sensor modes. Source: AMD

In addition to the three algorithms that determine how fan speeds respond to temperature changes, the SB800 series is governed by single- and dual-sensor policies that dictate how temperatures are interpreted. Single-sensor mode relies on only one temperature sensor, while the dual-sensor scheme takes in two inputs and will set the fan speed based on the higher of the two. Dual-sensor mode is designed primarily for motherboards with passively cooled north-bridge chips that rely on airflow generated by the CPU fan. In those cases, the thermal diode built into AMD's north-bridge silicon can be used as a secondary temperature input.


A view of the SB800 series' fan control inputs and outputs. Source: AMD

The SB800 series has inputs for three thermal diodes alongside the SMBus interface it uses to read the CPU's digital temperature sensor. Why so many? Because the south bridge can monitor and power no fewer than five individual fans. Each one can have its own profile, and 3- and 4-pin fans are supported across the board.

All this functionality is offered for free in every SB800-series south bridge, with no additional hardware required. Motherboard makers need only to connect fan and diode traces to the south bridge and implement the necessary BIOS hooks to give users access to fan control variables. But none have. AMD's Ali Akbar Merrikh, the man who designed the fan speed controls and patiently answered my questions about them, isn't aware of any motherboard that exploits the SB800's fancy fan controller outside the company's own boards and reference designs, none of which are available to consumers. That's a travesty, because this is exactly the sort of fan speed control that I'm sure many an enthusiast would love to have in his motherboard's BIOS. In fact, I'd wager that most of us would happily trade the ability to tweak obscure memory timings and system voltages for such robust fan control functionality.

Props to Merrikh and AMD for incorporating such powerful fan control support in the south bridge, and shame on any motherboard maker that leaves this particular feature untapped in favor of alternative fan control mechanisms that offer less, well, control.TR

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Tags: Motherboards Chipsets Cooling