Freescale Semiconductor TWR-MCF52259-Ethenet - Manual (Page 111)

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The General USB Process
Packets in More Detail
Packets are a block of information with a defined data structure. The packet is the lowest level of
the USB transfer hierarchy describing the physical layer of the interface. If you were to monitor
D+ and D- you would see the packet fields:
Packet Identifier
Address
Endpoint
Data
Frame number
CRC
5 4 7 8 Bits
CRC Endpoint Address PID Field
Token Packet format:
5 11 8 Bits
CRC Frame Number PID Field
SOF Packet format:
16 0-1023 8 Bits
CRC Data PID Field
Data Packet format:
8 Bits
PID Field
Handshake Packet format:
CRC covers everything in the packet with the exception of the PID which
has its own error checking mechanism.