82801DB Datasheet, PDF (185/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
Table 5-67. PID Format
Bit
0
1
2
3
Data Sent
PID 0
PID 1
PID 2
PID 3
Bit
Data Sent
4
NOT(PID 0)
5
NOT(PID 1)
6
NOT(PID 2)
7
NOT(PID 3)
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 5-67.
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 5-68.
Table 5-68. PID Types
PID Type
Token
PID Name
OUT
IN
SOF
SETUP
Data
Handshake
DATA0
DATA1
ACK
NAK
STALL
Special
PRE
PID[3:0]
b0001
b1001
b0101
b1101
b0011
b1011
b0010
b1010
b1110
b1100
Description
Address + endpoint number in host -> function transaction
Address + endpoint number in function -> host transaction
Start of frame marker and frame number
Address + endpoint number in host -> function transaction for
setup to a control endpoint
Data packet PID even
Data packet PID odd
Receiver accepts error free data packet
Rx device cannot accept data or Tx device cannot send data
Endpoint is stalled
Host-issued preamble. Enables downstream bus traffic to
low speed devices.
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.
Intel^® 82801DB ICH4 Datasheet
185