"AMD's SB800-series fan speed controller has the sort of functionality enthusiasts want in spades", apparently, and "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." More on these ignored fantastic features in the AMD chipset, in a rant by Geoff at The Tech Report. * Discuss this news in the SPCR forums.
"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.
"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.
"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."