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Appendix A USB Glossary

1. Bulk Transfers: One of four data transfer types on the USB bus. Bulk transfers are designed for

moving high volumes of data. They are capable of using any free bandwidth on the bus (that is,

bandwidth not already used by the other transfer types). This allows them to achieve the highest data

rates; but they are given no reserved bandwidth, so on a busy bus, bulk transfers might receive small

bandwidth or experience high latency. Transfer types are determined by the choice of USB interface

type; for example, CDC and MSC interfaces use bulk transfers.

2. Composite USB Device: A physical USB device (one USB connector) that contains more than one

USB interface – for example, two CDC interfaces or CDC+HID. The host enumerates each interface as

a separate logical entity.

3. Control Transfers: One of four data transfer types on the USB bus. Control transfers handle the

administrative tasks of setting up the connection, like reporting USB descriptors. The host also sends

other USB device requests, and the device responds using control transfers. There is a USB endpoint

dedicated for these transfers: endpoint 0 (EP0).

4. Device Class: A defined USB protocol for a particular class of devices. Common device classes

include the Communications Device Class (CDC), Human Interface Device (HID) class, and Mass

Storage Class (MSC).

5. Device Installation: The first time a USB device is enumerated, the host may perform one-time

functions to install the device. For example, Windows records information about the device in the

system registry, using the device's VID and PID as an index. In subsequent enumerations, the host

draws from the registry for much of its information about the device. Device installation may be silent

(mostly invisible to the end user) or, in the case of CDC on Windows, may require user action.

6. Endpoint: The end of a pipe. It acts as a "mailbox" on the USB device for that pipe. A device usually

has more than one active endpoint. When the host communicates on the bus, it first identifies the

physical USB device, then the endpoint number within that device that it wishes to speak to. Endpoints

are assigned specific functions according to the USB interfaces that were created. HID/MSC each use

one IN and one OUT endpoint, while CDC uses two IN and one OUT endpoint. In the MSP430 API

stacks, endpoint management is fully automated by the Descriptor Tool.

7. Enumeration: The process by which a host interrogates a physical USB device to determine what it is

and loads an appropriate driver so that the host application can interface with it. Enumeration happens

every time the device is attached.

8. Interrupt Transfers: One of the four USB data transfer types. Interrupt transfers are designed for

guaranteed latency, bandwidth, and delivery. However, the bandwidth is limited to only a single USB

packet (64 bytes for full-speed USB) per frame (1 ms). Transfer types are determined by the choice of

USB interface type; for example, HID interfaces use interrupt transfers.

9. Isochronous Transfers: One of the four USB data transfer types. Isochronous transfers provide

guaranteed latency and bandwidth but not delivery. That is, if error checking shows corrupted data, the

attempt is not retried. This type is intended for streaming audio and video -- applications in which a

retry would result in an interruption and thus be more noticeable to the user than simply missing the

packet.

10. INF (*.inf) file: A text-based file required during any USB device installation on Windows, allowing

Windows to associate the device with a particular driver. For some device classes, Windows contains

the INF internally, allowing for a silent device installation. For CDC, Windows prompts the end user for

the INF file.

11. Pipe: A single line of communication between host and device. Pipes are either IN (into the host) or

OUT (out of the host). They are characterized by a particular transfer type (for example, bulk or

interrupt).

12. Product ID (PID): A unique 16-bit value assigned by a USB hardware vendor to one of its products. A

VID and PID pair uniquely identifies a product type. (As a rule of thumb, if the USB descriptors of two

products differ in any way, they should have different PIDs.)

13. USB-IF: The USB Implementers Forum. This is the standards body that defines USB specifications,

governs USB certification, runs compliance workshops, and owns the legal rights to the USB logo.

14. USB Host: USB is hierarchal, with one (and only one) host that controls all communication.

SLAA457A–September 2013–Revised May 2014 Starting a USB Design Using MSP430™ MCUs

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