Technology Primer - USB Technology

Tech Primer

USB is a replacement for legacy serial & parallel communications and is a point to point connection including multi-drop topologies. The cabling consists of a pair of twisted wires carrying differential signalling data bundled with two wires for powering low-powered peripherals. A single cable of up to 5m long is allowed and for full and high-speed use it must be screened. Terminations are usually by the original standard ‘A’ and ‘B’ connectors with mini and micro versions exist and for some applications the device end may be permanently fixed to the PCB, a mouse being a good example.

• One end of the cable is the upstream host (master), the other the downstream function (slave) and unless we’re talking about USB-OTG (on-the-go) that    demarcation is cast in stone (well silicon).

• A beauty of USB is that for small, simple low-powered peripherals the power can be supplied by the host down the same cable (bus powered). The limit being 100mA before enumeration, 500mA after (see USB spec for more details).

• Device functions may be plugged / unplugged while the host remains powered, known as hot plugging. There are software exceptions to this (the hardware won’t mind), for example a mass storage device may not have completed it’s write process, the user must therefore wait until notified it is safe to unplug.

• Up to 127 functions are allowed per host port by fanning out into a tree structure using hubs, note the hubs count towards the 127 limit. There is of course a maximum supply current limit of 500mA so such a structure would almost certainly require "self-powered" hubs.

• USB-2.0 specifies speeds of:

“ Original” or “Low-speed“ at 1.5Mbits/s, the specification suggests a maximum useful data rate of 187KB/s max though many low-end micros won’t reach such sustained levels.

“ Full speed” at 12Mbits/s, for which the specification suggests a useful throughput of 1.5MB/s max

“ High speed” with 480Mbits/s yielding up to 60MB/s max of useful data throughput.

• The USB-OTG “on-the-go” extension was introduced to allow the host – function roles to be arbitrated. Taking the example of a printer, it may operate as a PC peripheral or as a camera host.

• Wireless USB and USB 3.0 have since been added but are sufficiently out of scope they’ll not be mentioned again here.

• The host controls the flow of data, no device function may send data back to the host without it being requested.

• Data is transferred between endpoints through “pipes”, think FIFOs, one or more at each end of the cable.

• Four transfer modes are employed:

• Bulk transfers are, as the name suggests, used to move large chunks of data, they have a low priority thus bandwidth may suffer in a system with isochronous and interrupt traffic.

• Isochronous transfers are used where regular data is required, for example streaming audio, each packet being up to about 1K.

• Interrupt transfers are a bit of a misnomer to anyone familiar with microprocessor design, these just guaranteed latency (regular) packets. The function doesn’t interrupt the host, rather the host polls it at a pre-arranged rate

• Control: used for setting up the function, these are short packets.

• Enumeration is in simple terms the process by which the host determines what has been plugged in and the subsequent configuration of it. The unit will for example have to report the vendor and product ID, allowing the host to load suitable device drivers.

• To be allowed to badge and sell USB peripherals the unit must undergo and pass compliance testing.

Other Resources

• How USB Ports Work
• USB 2.0, Hi Speed USB FAQ
• USB 3.0 Developers FAQ<
• USB Complete:The Developer's Guide, Fourth Edition
• USB Mass Storage Designing and Programming Devices and Embedded Hosts
• Universal Host Controller Interface (UHCI) Design Guide
• Inside USB 2.0: What the New Spec Means for Developers
• OpenHCI -- Open Host Controller Interface Specification for USB
• An introduction to Wireless USB (WUSB)
• Challenges of Migrating to Wireless USB
• Certified Wireless USB FAQ
• An Introduction to USB Development
• Migrating document from RS-232 UART to USB with minimal impact on PC software
• USB stack software and Embedded Host Software

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