Token packets

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Token packets

Post  meodingu on Mon Jan 10, 2011 8:38 am

Token packets
Token packets consist of a PID byte followed by 2 payload bytes: 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device.
IN and OUT tokens contain a 7-bit device number and 4-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively.
An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.
An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, NYET, or STALL, as appropriate.
SETUP operates much like an OUT token, but is used for initial device setup. It is followed by an 8-byte DATA0 frame with a standardized format.
Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-bandwidth USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 Ás "microframe" (60000 high-bandwidth bit times each).
USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK.
USB 2.0 also added a larger 3-byte SPLIT token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-bandwidth USB bus sending data to a slower USB device, the nearest high-bandwidth capable hub receives a SPLIT token followed by one or two USB packets at high bandwidth, performs the data transfer at full or low bandwidth, and provides the response at high bandwidth when prompted by a second SPLIT token. The details are complex; see the USB specification.

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