82801BA Datasheet, PDF (182/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.16.4 Bus Protocol
5.16.4.1
Bit Ordering
Bits are sent out onto the bus least significant bit (LSb) first, followed by next LSb, through to the
most significant bit (MSb) last.
5.16.4.2
SYNC Field
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 first bit of the PID. All subsequent bits in
the packet must be indexed from this point.
5.16.4.3 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.
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.
5-120
82801BA ICH2 and 82801BAM ICH2-M Datasheet