82801BA Datasheet, PDF (185/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.16.5 Packet Formats
5.16.5.1
Token Packets
Table 5-71 shows the field formats for a token packet. A token consists of a PID, specifying either
IN, OUT, or SETUP packet type, and ADDR and ENDP fields. For OUT and SETUP transactions,
the address and endpoint fields uniquely identify the endpoint that will receive the subsequent data
packet. For IN transactions, these fields uniquely identify which endpoint should transmit a data
packet. Only the ICH2 can issue token packets. IN PIDs define a data transaction from a function
to the ICH2. OUT and SETUP PIDs define data transactions from the ICH2 to a function.
Token packets have a five-bit CRC that covers the address and endpoint fields as shown above. The
CRC does not cover the PID, which has its own check field. Token and SOF packets are delimited
by an EOP after three bytes of packet field data. If a packet decodes as an otherwise valid token or
SOF but does not terminate with an EOP after three bytes, it must be considered invalid and
ignored by the receiver.
Table 5-71. Token Format
Packet
PID
ADDR
ENDP
CRC5
Width
8 bits
7 bits
4 bits
5 bits
5.16.5.2 Start of Frame Packets
Table 5-72 shows a start of frame (SOF) packet. SOF packets are issued by the host at a nominal
rate of once every 1.00 ms. SOF packets consist of a PID indicating packet type followed by an 11-
bit frame number field.
The SOF token comprises the token-only transaction that distributes a start of frame marker and
accompanying frame number at precisely timed intervals corresponding to the start of each frame.
All full speed functions, including hubs, must receive and decode the SOF packet. The SOF token
does not cause any receiving function to generate a return packet; therefore, SOF delivery to any
given function cannot be guaranteed. The SOF packet delivers two pieces of timing information. A
function is informed that a start of frame has occurred when it detects the SOF PID. Frame timing
sensitive functions, that do not need to keep track of frame number, need only decode the SOF PID;
they can ignore the frame number and its CRC. If a function needs to track frame number, it must
comprehend both the PID and the time stamp.
Table 5-72. SOF Packet
Packet
PID
Frame Number
CRC5
Width
8 bits
11 bits
5 bits
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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