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82801AA and 82801AB Datasheet
5-87
Functional Description
5.15.4.3
Packet Field Formats
Field formats for the token, data, and handshake packets are described in the following section. The
effects of NRZI coding and bit stuffing have been removed for the sake of clarity. All packets have
distinct start and end of packet delimiters.
Packet Identifier Field
A packet identifier (PID) immediately follows the SYNC field of every USB packet. A PID
consists of a four bit packet type field followed by a four-bit check field as shown in
Table 5-65
.
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 four-bit check field of the PID insures reliable decoding
of the PID so that the remainder of the packet is interpreted correctly. The PID check field is
generated by performing a ones complement of the packet type field.
Any PID received with a failed check field or which decodes to a non-defined value is assumed to
be corrupted and the remainder of the packet is assumed to be corrupted and is ignored by the
receiver. PID types, codes, and descriptions are listed in
Table 5-66
.
PIDs are divided into four coding groups: token, data, handshake, and special, with the first two
transmitted PID bits (PID[1:0]) indicating which group. This accounts for the distribution of PID
codes.
Table 5-65. PID Format
Bit
Data Sent
Bit
Data Sent
0
PID 0
4
NOT(PID 0)
1
PID 1
5
NOT(PID 1)
2
PID 2
6
NOT(PID 2)
3
PID 3
7
NOT(PID 3)
Table 5-66. PID Types
PID Type
PID Name
PID[3:0]
Description
Token
OUT
b0001
Address + endpoint number in host -> function transaction
IN
b1001
Address + endpoint number in function -> host transaction
SOF
b0101
Start of frame marker and frame number
SETUP
b1101
Address + endpoint number in host -> function transaction
for setup to a control endpoint
Data
DATA0
b0011
Data packet PID even
DATA1
b1011
Data packet PID odd
Handshake
ACK
b0010
Receiver accepts error free data packet
NAK
b1010
Rx device cannot accept data or Tx device cannot send
data
STALL
b1110
Endpoint is stalled
Special
PRE
b1100
Host-issued preamble. Enables downstream bus traffic to
low speed devices.