The SIE logic manages the USB packet protocol requirements for the packets being received and transmitted on the bus. For packets being received, the SIE decodes the packet identifier field (packet ID) to determine the type of packet being received and to ensure the packet ID is valid. For token and data packets being received, the SIE calculates the packet cycle redundancy check (CRC) and compares the value to the CRC contained in the packet to verify that the packet was not corrupted during transmission.
For token and data packets being transmitted, the SIE generates the CRC that is transmitted with the packet. For packets being transmitted, the SIE also generates the synchronization field (SYNC), which is an eight-bit field at the beginning of each packet. In addition, the SIE generates the correct packet ID for all packets being transmitted. Another major function of the SIE is the overall serial-to-parallel conversion of the data packets being received or transmitted.