May 2005
Beginner to intermediate
398 pages
12h 12m
English
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USB Packets
There are four types of transfers that can take place over USB. A control transfer is used to configure the bus and devices on the bus, and to return status information. A bulk transfer moves data asynchronously over USB. An isochronous transfer is used for moving time-critical data, such as audio data destined for an output device. Unlike a bulk transfer, which can be bidirectional, an isochronous transfer is uni-directional and does not include a cyclic-redundancy-check (CRC) field. An interrupt transfer is used to retrieve data at regular intervals, ranging from 1 to 255 milliseconds.
Data is transferred between USB devices using packets, and a transfer can comprise one or more packets. A packet consists of a SYNC (synchronization) byte, a PID (Packet ID), content (data, address, etc.), and a CRC.
The SYNC byte phase locks the receiver's clock. This is equivalent to the start bit of an RS-232C frame. The PID indicates the function of the packet, such as whether it is a data packet or a setup packet. The upper four bits of the packet ID are the inverse of the lower four bits, for additional error checking. For example, the packet ID for a data packet is 0x3C. In binary, this is %0011 1100.
USB packets can be one of four types: token, data, handshaking, or preamble.
Tokens are 24-bit packets that determine the type of transfer that is to take place over the bus. There are four types of token packet (Figure 11-3). A token packet consists of a SYNC byte, ...
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