Section 24.2. USB Host Controller Interfaces


24.2 USB Host Controller Interfaces

The intent of USB was to provide a standardized Plug-N-Play interface that would permit USB-compliant devices from any manufacturer to coexist peacefully with those from any other maker. The ball got rolling in September 1995, when 25 major PC companies announced the formation of a consortium to develop a non-proprietary, open Host Controller Interface (HCI) standard. This was necessary because the HCI was not defined in the USB specification itself. Although it would have been possible for each motherboard or chipset manufacturer to develop a proprietary USB HCI, that would have introduced serious compatibility issues between the multiple HCIs and USB peripherals. So, for the good of the industry and in the interests of interoperability, nearly all of the major players in the PC industry signed on to this initiative.

The HCI is the heart of USB, just as the chipset is the heart of a motherboard. In fact, the USB HCI is often referred to as the USB chipset. Just as the system chipset defines the functionality and capabilities of the motherboard, coordinates the working of other motherboard components, and arbitrates conflicting demands, a USB HCI chipset defines the USB and performs analogous services for connected USB peripherals.

The HCI may reside in any or all of three places:

Embedded in the motherboard chipset

All modern chipsets contain a USB 1.1 HCI embedded in the Southbridge. The quality and compatibility of the embedded HCI depends on the maker of the chipset and its age, but even the best and most recent embedded USB 1.1 HCIs may have issues with some peripherals, particularly older models. If you use USB heavily, and especially if you use many peripherals, it's often a good idea to disable the embedded HCI in BIOS and substitute an add-on PCI USB controller of more recent vintage. Although Intel shipped some motherboards with native USB 2.0 support beginning in early 2002, those used a supplementary NEC chip to provide USB 2.0 support. The first production chipset with embedded USB 2.0 support was the Intel 845G-series, which began shipping in May 2002. As of June 2002, no other production chipsets include USB 2.0 support, although most chipsets introduced from mid-2002 onward will include USB 2.0.

As a separate chip on the motherboard

A few motherboards implement the HCI as a discrete chip, separate from the main chipset Southbridge. This method is common in USB 2.0 transition motherboards. For example, the Intel 845 chipset used in the D845BGSE motherboard supports only USB 1.1 natively, so Intel uses a separate NEC mPD720100 USB 2.0 host controller as a stopgap to add USB 2.0 functionality to this motherboard until Intel chipsets with integrated USB 2.0 functionality are available. As with embedded USB HCIs, a discrete HCI can be disabled from BIOS Setup should that become necessary.

On an add-on peripheral card

Add-on PCI USB adapters are available from numerous makers in both USB 1.1 and USB 2.0 variants. USB 2.0 adapters cost only a few dollars more than USB 1.1 adapters, so if you need to add a USB adapter, we recommend buying only a USB 2.0 model. As of June 2002, all USB 2.0 adapters that we know of use the NEC mPD720100 host controller, which fortunately seems to be a good one.

If you buy a USB 2.0 adapter, make sure your operating system supports it as a USB 2.0 device. In particular, Windows 2000 support is problematic. Microsoft plans to ship native Windows 2000 USB 2.0 drivers by mid-2002, but unless and until that happens, a USB 2.0 adapter functions only as a USB 1.1 adapter under Windows 2000. The sole exceptions that we know of are the Adaptec and Belkin USB 2.0 adapters, which have their own Windows 2000 drivers. Most USB 2.0 adapter makers claim Windows 2000 compatibility, but note in the fine print that they require the (nonexistent) Windows 2000 USB 2.0 driver to do so. Although we expect that Microsoft will in fact release Windows 2000 USB 2.0 drivers, they have changed their minds before.

It is becoming increasingly common for motherboards to include more than one HCI. Because each HCI defines a USB, motherboards with multiple HCIs actually have two or more USBs defined, each of which has its own root hub ports and operates independently from the others. The benefits to having multiple USBs are that you can segregate devices that conflict with each other by connecting them to separate USBs, that you can dedicate separate USBs to USB 1.1 versus USB 2.0 devices, and that multiple HCIs allow the system to have several USB ports available at both the front and rear of the case.

24.2.1 USB Host Controller Interface Types

There are three types of USB host controller:

Open Host Controller Interface (OHCI)

The Open Host Controller Interface (OHCI) is a USB 1.x interface that was developed by a consortium of PC makers led by Compaq, Microsoft, and National Semiconductor. OHCI places more of the burden for managing USB functions on hardware, and less on software. OCHI is most common in systems that use non-Intel chipsets, including those from ALi, SiS, and VIA. Windows 98/SE/Me and Windows 2000/XP include native OHCI drivers. You can download the full Open Host Controller Interface Specification from

If you run Windows 98SE on a system with an OCHI, visit the Microsoft web site and read Knowledge Base article Q253697. This combination of OS and HCI can cause some very odd behavior, such as unplugging a bulk-mode device from one root hub port causing a hub connected to a different root hub port to disconnect. One possible symptom, believe it or not, is your computer making a high-pitched whining noise, like a phaser about to self-destruct.

Universal Host Controller Interface (UHCI)

The Universal Host Controller Interface (UHCI) is a USB 1.x interface that was developed by Intel. UHCI places more of the burden for managing USB functions on software, and less on hardware. UCHI is most common in systems with Intel chipsets, although many non-Intel chipsets also use UHCI. USB devices designed to use UHCI (e.g., the Intel USB camera) may not work properly with OHCI. Windows 98/SE/Me and Windows 2000/XP include native UHCI drivers. You can download the full Universal Host Controller Interface Design Guide from

Enhanced Host Controller Interface (EHCI)

The Enhanced Host Controller Interface (EHCI) is the single USB 2.0 interface, used by all USB 2.0 devices. The simple fact that a single integrated HCI specification exists for USB 2.0 should eliminate many of the problems that occurred with USB 1.1 because of competing HCI standards. Of Microsoft operating systems, only Windows XP has native EHCI drivers available. Windows 2000 EHCI drivers are to be released by mid-2002. Microsoft has no plans to release EHCI drivers for any variant of Windows 9X. You can download the full Enhanced Host Controller Interface Specification from

24.2.2 USB Host Controller Interface Issues

In addition to the standard they support OHCI, UHCI, or EHCI USB host controllers vary in how well they are implemented. Early host controllers from all manufacturers, including Intel, have numerous bugs and incompatibilities, which may manifest in various ways, from minor performance glitches to a complete failure of the USB to recognize devices.

It is well to be aware of the particular host controller your system uses. That information can save you trouble when you are attempting to connect various devices to your system, and may even help you avoid buying a new USB device that has known incompatibilities with your existing configuration.

Here are details about some of the USB host controllers you are likely to encounter:

ALi chipsets

Although we do not have extensive experience with ALi chipsets, there have been enough reported problems that we consider USB support to be suspect. When configuring an ALi-based system to use USB, we recommend updating the main system BIOS to the most recent available, installing the latest chipset drivers, disabling ACPI and power management in BIOS, and setting the BIOS PnP OS? option to No. In Windows, replace the ALi USB Host Controller with the Windows Standard OpenHCI USB Host Controller, and disable USB error detection (for Win98SE or later only). If your motherboard has an old ALi chipset and you encounter USB problems, often the most practical solution is to install a PCI/USB card.

AMD chipsets

The AMD-750 "Irongate" chipset has reasonably robust USB support for an older chipset. Microsoft Knowledge Base article Q241134 describes one known problem that occurs under Windows 98 and 98SE with low-speed devices such as mice and keyboards, and provides a patch for that problem. The AMD-760 and AMD-760MP provide generally reliable USB support, although we recommend running motherboards based on those chipsets with ACPI disabled. Although the first stepping of the AMD-760MPX chipset had a widely reported USB bug, we expect later steppings of that chipset to resolve the problem.

Intel PIIX3 (82371SB)

The PIIX3, used in the Intel 430HX Pentium chipset, is USB 1.0 compliant and does not function properly with many USB 1.1 devices. The PIIX3 supports one USB 1.0 UHCI and two root hub ports. This chipset (and the motherboards that use it) is so ancient that you should not even consider attempting to connect modern USB devices to it. If upgrading the motherboard is not an option, install an add-on USB card such as those sold by Adaptec. If for some reason you must use the 82371SB, you may be able to resolve some problems by substituting the 82317AB or 82371AB/EB driver.

Technically, any USB 1.1-compliant peripheral should be compatible with any USB 1.0-compliant controller. The updates from the USB 1.0 Specification to USB 1.1 tightened up ambiguities on the peripheral side to eliminate conflicts, but made only minor changes to the host-side specification. Conflicts between USB 1.0 controllers and USB 1.1 peripherals are due less to the difference in USB specification level than to bugs in USB 1.0 controllers.

Intel PIIX4 (82371AB)

The PIIX4, used in the Intel 430TX chipset, is USB 1.1 compliant, despite the fact that its datasheet claims only USB 1.0 compliance. The PIIX4 supports one USB 1.1 UHCI and two root hub ports. Although this chipset should in theory support modern USB 1.1 devices, it is quite buggy and we therefore recommend not attempting to use it. Again, replace the motherboard or install a modern add-on USB adapter. If for some reason you must use the 82371AB, you may be able to resolve some problems by substituting the updated 82371AB/EB driver.

For the Intel PIIX4 and later Intel chipsets, we suggest as a first step that you visit and download the most recent Intel Chipset Software Installation Utility. Heed all cautions listed.

Intel PIIX4E (82371EB)

The PIIX4E is used in the Intel 440BX, 400EX, 440LX, 440ZX, 440ZX66, 440GX, and 450NX chipsets, and is the oldest USB host controller that you should consider using in a production system. The PIIX4E supports one USB 1.1 UHCI and two root hub ports, and has many fewer compatibility problems than do earlier Intel USB controllers. The most common problem with the PIIX4E is a device not being recognized when you connect it. The PIIX4E also may become confused and lose connectivity if the screensaver kicks in or if the system enters standby, suspend, or hibernate mode. If that occurs, hot-plugging the device (disconnecting and then reconnecting the USB cable while the PC and device remain on) may resolve the problem. In our experience, the 82371EB seems reasonably well behaved with low-speed USB devices like mice and keyboards, particularly under Windows 2000. It is more likely to have problems with high-speed and bulk-mode USB devices, particularly scanners, printers, and digital cameras, and particularly under Windows 9X.

Intel I/O Controller Hub (ICH) (82801AA) and I/O Controller Hub (ICH0) (82801AB)

The 82801AA I/O Controller Hub (ICH) and 82801AB I/O Controller Hub (ICH0) are used in the Intel 810, 810E, 815, 815G, 815P, 820, and 840 chipsets. The 82801AA and 82801AB differ primarily in physical packaging, although different drivers have been released for them. The 82801AA and 82801AB support one USB 1.1 UHCI and two root hub ports. Although the 82801AA and 82801AB have significantly fewer incompatibilities than earlier Intel USB controllers, you may still experience intermittent connection problems. If you have problems with the 82801AA, try installing the 82801AB driver, which is fully compatible with the 82801AA but has fewer issues.

Intel I/O Controller Hub 2 (ICH2) (82801BA/BAM)

The 82801BA/BAM I/O Controller Hub 2 (ICH2) is used in the Intel 810E2, 815E, 815EG, 815EP, 820E, 845, 850, and 860 chipsets. The 82801BAM is the mobile version of the 82801BA. The 82801BA/BAM supports two USB 1.1 UHCIs and four root hub ports. We have experienced no USB problems attributable to this chipset, and reported problems are relatively rare. If you do experience problems, try substituting the 82801AB driver.

If you are experiencing USB problems, the fix may be as simple as upgrading the main system BIOS, particularly if your USB controller is a recent model. Even if it is not, however, a BIOS upgrade may solve or minimize USB conflicts. For example, we have an elderly Intel SE440BX-2V motherboard with an 82371EB USB host controller.Attempting to use an Epson inkjet printer and a Umax scanner on that system was an exercise in frustration, with apparently random problems plaguing us. For no apparent reason, one or the other device would stop working. When both were working, powering off or disconnecting one device sometimes caused the other to fail to be recognized.

Attempting to use the scanner while the printer was printing would sometimes lock up one or both devices. When we rebooted the system, sometimes only the printer would appear in Device Manager; other times, only the scanner would appear. Sometimes there would be two copies of one or both devices in Device Manager. And so on. In other words, a not-untypical USB 1.1 experience.

The camel-back-breaking straw happened one day when we connected a USB mouse. That worked fine, with one exception. Every time we moved the mouse, the printer ejected a sheet of paper. Arrghhh. Enough was enough. We finally installed a BIOS upgrade on the SE440BX-2V, and many of the problems became less severe. Then we stripped the hard drive down to bare metal and reinstalled the operating system. That pretty much cured the problem, although we still keep our fingers crossed.

SiS chipsets

In the past, we avoided SiS chipsets, as we have had numerous bad experiences with them, including what appeared to be irresolvable USB conflicts. We still avoid early SiS chipsets, but, as the old saying goes, even a blind pig finds an acorn occasionally. The SiS735 chipset for AMD processors is a solid chipset. It is inexpensive, fast, and appears to be robust in all respects, including USB support. We expect that the follow-on SiS745 chipset will be just as good, although we haven't yet tested it. When configuring older SiS chipsets to use USB, we recommend updating the main system BIOS to the most recent available, installing the latest chipset drivers, disabling ACPI and power management in BIOS, and setting the BIOS PnP OS? option to No. In Windows, replace the SiS PCI to USB Open Host Controller with the Windows Standard OpenHCI USB Host Controller, and disable USB error detection (for Win98SE or later only). If your motherboard has an old SiS chipset and you encounter USB problems, often the only real solution is to install a PCI/USB card.

VIA chipsets

We consider USB support in older VIA chipsets to be fundamentally flawed. Even recent VIA chipsets for both Intel and AMD processors have USB issues, although problems are most severe with VIA chipsets from 1999 and earlier. In our experience, system hangs, stop errors, and dropped connections are common with VIA chipsets, particularly if you use high-speed bulk-mode USB peripherals. Unfortunately, the VIA web site is not forthcoming about USB issues, attributing problems primarily to operating system and power issues rather than to incompatibilities within the chipsets themselves. We recommend you avoid running USB on any VIA-based motherboard older than a year or two if at all possible, and also that you avoid VIA-based PCI/USB cards. If you run USB on any VIA chipset and experience USB problems, download and install the latest USB Filter Patch from Better still, install a PCI/USB card and stop using the motherboard USB ports.

There is a severe USB bug in Windows 98SE (Windows 98, Me, and 2000/XP are not affected) running on systems with a VIA chipset and an AMD processor running faster than 350 MHz. See the Microsoft Knowledge Base article Q240075 for details and a patch file.

Windows 2000 and XP do not support VIA chipsets REV_04 or earlier. Windows 2000 explicitly does not support VIA chipsets with the VT83C572 or VT82C586 chips. Microsoft warns that these chipsets are irreparably flawed, that they cannot be fixed in software, and that using them may cause system instability or data loss.


    PC Hardware in a Nutshell
    PC Hardware in a Nutshell, 3rd Edition
    ISBN: 059600513X
    EAN: 2147483647
    Year: 2002
    Pages: 246 © 2008-2017.
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