Packet Formats - Samsung S3C2500B User Manual

USB CONTROLLER

10.3.3 PACKET FORMATS

All packets begin with a synchronization (SYNC) field, which is a coded sequence that generates a maximum
edge transition density. The SYNC field appears on the bus as IDLE followed by the binary string "KJKJKJKK", in
its NRZI encoding. It is used by the input circuitry to align incoming data with the local clock and is defined to be
eight bits in length. SYNC serves only as a synchronization mechanism and is not shown in the following packet
diagrams. The last two bits in the SYNC field are a marker that is used to identify the end of the SYNC field and,
by inference, the start of the PID.
The PID indicates the type of packet and, by inference, the format of the packet and the type of error detection
applied to the packet. The host and all functions must perform a complete decoding of all received PID fields.
Any PID received with a failed check field or which decodes to a non-defined value is assumed to be corrupted
and it, as well as the remainder of the packet, is ignored by the packet receiver. If a function receives an
otherwise valid PID for a transaction type or direction that it does not support, the function must not respond. For
example, an IN-only endpoint must ignore an OUT token.
Function endpoints are addressed using two fields: the function address field and the endpoint field. The function
address (ADDR) field specifies the function, via its address, that is either the source or destination of a data
packet, depending on the value of the token PID.
Cyclic redundancy checks (CRCs) are used to protect all non-PID fields in token and data packets. In this
context, these fields are considered to be protected fields. The PID is not included in the CRC check of a packet
containing a CRC. All CRCs are generated over their respective fields in the transmitter before bit stuffing is
performed.
Handshake packets are used to report the status of a data transaction and can return values indicating successful
reception of data, command acceptance or rejection, flow control, and halt conditions.
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Figure 10-2. USB 1.1 Frame Model
Low Speed BULK
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S3C2500B