The A55 & A75 Fusion Controller Hub (FCH)
The A55 & A75 Fusion Controller Hub (FCH)
Prior to Llano, AMD used a two chip Northbridge / Southbridge solution on their motherboards AMD’s 890FX Chipset: An Evolution for the AM3 Platform where the Northbridge was responsible for tasks like supplying the primary PCI-E lanes, facilitating communication between the Southbridge and the CPU and providing a platform for onboard graphics on some chipsets. The Southbridge meanwhile was used primarily as an I/O hub for USB, SATA, audio and networking connections while supplying a few additional PCI / PCI-E lane for general purpose connectivity. With Llano and the FM1 socket things have changed somewhat as all of the Northbridge’s functions are now built directly into the CPU die
With the Northbridge’s functionality built directly into the APU, the general layout of A-series motherboards begins to look a lot like Intel’s have since Clarkdale was introduced. The DDR3 memory controller, display outputs for the graphics controller and 16 PCI-E lanes for discrete graphics cards originate from the APU die. This x16 layout can be split into two x8 slots for Crossfire support but for the time being SLI hasn’t been certified for AMD’s APU motherboards. Four additional PCI-E 2.0 lanes for General Purpose Ports also originate from the APU and are used for offloading high demand I/O functions from the UMI interface.
AMD has transferred the typical Southbridge functions into an all-in-one solution called the Fusion Controller Hub of which there are two models: the A75 and lower end A55. Connecting the FCH to the APU is the Unified Media Interface (or UMI) which consists of four PCI-E 2.0 lanes for I/O transfers and system management. This results in an interconnect bandwidth of 2 GB/s which is a far cry from the 4.16 GB/s (5.2 GT/s) of the previous generation’s Northbridge / CPU pathway. However, the high bandwidth of Hypertransport 3.0 between what amounts to a glorified Southbridge and the APU isn’t needed since the FCH contains none of the Northbridge’s demanding PCI-E lanes.
The Fusion Controller Hub is a deceptively simple 65nm chip which is responsible for a host of functions. It incorporates functionality for SATA, USB, HD audio in case the A/V outputs on the GPU aren’t used and controls up to three legacy PCI slots. One of the more interesting features which AMD has built in to the FCH is native compatibility with consumer IR (CIR) devices like remotes and this could prove to be a boon for HTPC users. The addition of a 16MB BIOS chip onto A75 and A55 boards means UEFI support is possible and some manufacturers like ASUS have implemented just that.
The differences between the A75 and A55 FCH may not seem all that apparent upon first glance but the lower end chips do have some key features missing. To begin with, the A75 FCH is the first controller on the market to natively support both USB 3.0 (four ports) and SATA 6 Gb/s (6 ports) while the A55 doesn’t natively support either format. Any board sporting an A55 FCH can still implement USB 3.0 or SATA 6 Gb/s but it will need somewhat expensive third party controller chips to do so. The only other real difference between these two chipsets is the A55’s lack of FIS Base Switching for RAID setups.
According to AMD, the implementation of native USB 3.0 ports running directly off of the A75 chipset will allow for higher performance levels than any current third party controller can supply due to a virtual elimination of processing overhead. Most users likely won’t feel a difference between native and non-native designs but there is a difference on paper if one was to look closely at performance.
One area in which AMD seems to have a clear advantage over Intel is within the display output selection. Currently, Intel’s onboard graphics solutions officially support resolutions of up to 1920 x 1200 (we have actually had issues getting this resolution to work on some Intel setups) through a single link DVI port or HDMI connector. AMD’s onboard graphics processor on the other hand fully supports resolutions up to 2560 x 1600 via DVI or DisplayPort.
In addition to compatibility with high resolution monitors, the Llano APUs have a flexible display interface which allows PCI-E lanes to be configured on the fly in order to expand the desktop onto a pair of screens if needed. The Fusion Controller Hub also has an integrated DAC which supports a single VGA display which can be used in lieu of precious PCI-E bandwidth.
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