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2) End of Packet (EOP)
Every packet is terminated by an end of packet (EOP) consisting of 2 transmitted single-ended
zeros (SE0) followed by a driven idle state for another bit time.
3) Packet Identifier (PID)
Each packet contains a packet identifier (PID) declaring the packet function. The PID is
transmitted as a 4-bit encoding followed by the bitwise inversion of the PID as an error check.
If the PID check fails, the packet is ignored since without the a valid PID the packet cannot be
identified. Table 18-14 shows the PID encoding and definitions for each packet type.
Table 18.14 PID Encoding
Packet Type
Function
PID[3:0]
Description
Token
OUT
0001
Host to Device transaction request
IN
1001
Device to Host transaction request
SOF
0101
Start of Frame timing mark
SETUP
1101
Host to Device Control transaction setup
Data
DATA0
0011
Even data packet
DATA1
1011
Odd data packet
Handshake
ACK
0010
Receiver accepts the data packet
NAK
1010
Receiver rejects the data packet or transmitter cannot
send data packet
STALL
1110
Endpoint is stalled.
Special
PRE
1100
Preamble for low speed driver enable in hubs
4) Token Packet
There are four types of token packets described below: IN, OUT, SETUP, Start of Frame
(SOF). IN and OUT specify data packet direction in the data phase of the transaction. SETUP
has the same meaning as an OUT token to the SIE but has special meaning to the device
decoding it. A SOF is used to synchronize all USB devices to a nominal 1 ms bus interval.
An IN, OUT, or SETUP token packet contains an 11-bit data field containing the device and
endpoint address. For an SOF token, this same 11-bit field contains the lower 11 bits of
FrameNumber in the HcFmNumber register. The data field is followed by a 5-bit CRC. The
root hub is the only device which can send a token packet so it is not necessary to decode a
token packet. All fields are serially transmitted LSb first (except the CRC).
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