Data packets - universal serial bus, Computer Engineering

Data packets:

A data packet consists of the PID which is followed a 16-bit CRC and by 0-1023 bytes of data payload (up to 1024 in high speed and at most 8 at low speed)

There are 2 basic data packets, DATA0 and DATA1. They have to always be preceded by an address token, and are typically followed by a handshake token from the receiver back to the transmitter. The 2 packet types provide the 1-bit sequence number needed by Stop-and-wait ARQ. If a USB host does not retain a response (such as an ACK) for data it has transmitted, it doesn't know if the data was retained or not; the data may have been lost in transit, or it may have been received but the handshake response was lost.

To solve this type of problem, the device keeps track of the type of DATA x packet it last accepted. If it retains another DATA x packet of the similar type, it is acknowledged but ignored as a case of duplicate. Only a DATA x packet of the opposite type is really received.

When a device is reset having a SETUP packet, it expects an 8-byte DATA0 packet next.

USB 2.0 added DATA2 and MDATA packet types as well. They are used just by high-speed devices doing high-bandwidth isochronous transfers which required transferring more than 1024 bytes per 125 ┬Ás "micro frame" (8192 kB/s),PRE "packet"

Low-speed components are supported with a special PID value, PRE. It marks the beginning of a low-speed packet, and it is used by hubs which usually do not send full-speed packets to low-speed devices. As all PID bytes include four 0 bits, they leave the bus in the FS(full speed)K state, which is the similar as the low-speed J state. It is followed by a deep pause during which hubs enable their low-speed outputs that is already idling in the J state, then a low-speed packet follows the starting with a sync sequence and PID byte, and ending with a deep period of SE0. Full-speed components other than hubs can simply avoid the PRE packet and its low-speed contents, till the final SE0 indicates that a new packet follows.

Posted Date: 10/13/2012 7:51:53 AM | Location : United States







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