document updated 16 years ago, on Apr 16, 2008

USB peripheral controllers for the PC

A USB host controller is the bit of hardware on a computer that allows it to act as the master in the master/slave relationship in USB communications.

A USB peripheral controller (or "device controller"), then, is the hardware that performs the USB slave communication. Usually these are integrated in/near microprocessors, and located inside very small devices like mice. However, full-fledged computers are sometimes able to communicate as a USB slave. This can be due to the fact that more and more peripherals are using embedded Linux/Windows, but just as often, it's because some USB device designers use a PC as an initial prototype.

Drivers

Linux's USB gadget API (see options)

Hardware (PC interface)

PLX NET2280EVB, the development/evaluation PCI card that showcases their NetChip controller ($100? $400? $800?)
USB OTG connections can act purely as a peripheral. OTG is actually a small modification of USB — it just allows devices to exchange master/slave roles. The important points for us are that an OTG interface's initial role is solely determined by which end of a cable is inserted (search for "cable based role switching"), and that USB OTG devices are 100% compliant with USB 2.0 devices when they stick with their initial role (search for "Q4") (this fact is particularly evident when you you hook an OTG device to a hub)
- On the other hand, OTG PC cards are even harder to obtain than Device Controller PC cards. (and the number of peripherals that implement it is rather small as well)

Hardware (chips)

While individual chips require extra effort and money to make them be able to effectively cooperate with a computer, it's still a fallback option to be aware of. (particularly if they ultimately make it clear that KVM designs should never have a massive PC wedged in the middle)

There are many microcontrollers and single-board computers that incorporate a USB device controller.
"Hub controllers" usually contain one upstream port, and 4-8 downstream ports. Very few USB controller chips contain multiple upstream ports, because that ALWAYS implies that beyond-spec switching/sharing is occuring (the spec says communications must form a directed tree). Multi-upstream chips are sometimes are called "KVM controllers" or "USB switch controllers". Specific chips include:
- Alcor AU9228, USB KVM controller
- SMC USB2524, USB MultiSwitch Hub (product brochure)
"OTG bridge controllers" are USB OTG controllers that can ferry mass-transfer requests from one to the other. Actually, the Philips ISP1261 is a WHOLE LOT more general than that. It has a "software emulated OTG controller" (SEOG) that can really open a lot of doors! (however... it can't directly hook to a PC, can it? The PC is expecting to talk to a peripheral, not a host... So this solves nothing?
A chip that contains only the PHY and the SIE is able to connect as either an upstream or downstream connection, and can function as a hub. Controller chips that contain the higher-level processing are sometimes limited to only a few ongoing endpoints, and thus aren't able to pass an arbitrary number of connections over them like a hub would.
On the other hand, if you write software that talks directly to an SIE, you have to implement higher-level parts of the USB protocol yourself (though this doesn't apply when you're simply relaying packets without modification or deeper inspection). If you look at functional block diagrams of USB hubs (one, two, three), there's definitely more to them than the PHY+SIE (particularly for USB2.0-compliant hubs, since they require separate transaction translators for each port).
Chips that fill this role include:
- any separate PHY chip that can be attached to a separate SIE chip, of course
- Maxim MAX3421E
- PIC24F*, PIC32*, or PIC18* microcontrollers (while they do support the full set of USB, only the SIE is implemented in hardware, the rest is in software only!)