NQ5000P S L9TN Intel, NQ5000P S L9TN Datasheet - Page 351

• Manufacturer Part Number: NQ5000P S L9TN
• Manufacturer: Intel

Specifications of NQ5000P S L9TN

• Mounting: Surface Mount
• Lead Free Status / RoHS Status: Not Compliant

Functional Description
Figure 5-23. PCI Express Packet Visibility By Link Layer
5.12.8.1
5.12.8.2
5.12.8.3
Intel® 5000P/5000V/5000Z Chipset Memory Controller Hub (MCH) Datasheet
Data Link Layer Packets (DLLP)
Refer to PCI Express Base Specification, Revision 1.0a for an explicit definition of all the
fields in a Data Link Layer packet.
DLLPs are used to ACK or NAK packets as they are sent from the transmitter to the
receiver. They are sent by the receivers of the packet to indicate to the transmitter that
a packet was successfully received (ACK) or not (NAK). DLLPs are also used to
exchange credit information between the transmitter and receiver.
DLLPs are protected with 16b CRC. If the CRC of a received DLLP indicates an error, the
DLLP is dropped. This is safe because the PCI Express protocol supports dropping these
packets and the next DLLP allows the transmitter to process successfully.
ACK/NAK
The Data Link layer is responsible for ensuring that packets are successfully
transmitted between PCI Express agents. PCI Express implements an ACK/NAK
protocol to accomplish this. Every packet is decoded by the physical layer and
forwarded to the link layer. The CRC code appended to the packet is then checked. If
this comparison fails, the packet is “retried”.
If the comparison is successful, an ACK is issued back to the transmitter and the packet
is forwarded for decoding by the receiver’s Transaction layer. Typically, as each packet
is successfully received by the Data Link layer, the receiver issues an ACK. However,
the PCI Express protocol allows that ACKs can be combined.
Link Level Retry
The PCI Express Base Specification, Revision 1.0a lists all the conditions where a
packet gets negative acknowledged. One example is on a CRC error. The link layer in
the receiver is responsible for calculating 32 b CRC (using the polynomial defined in the
PCI Express Base Specification, Revision 1.0a) for incoming packets and comparing the
calculated CRC with the received CRC. If they do not match, then the packet is retried
by negative acknowledging the packet with a NAK DLLP and specifying the sequence
number of the last good packet. Subsequent packets are dropped until the reattempted
packet is observed again.
When the transmitter receives the NAK, it is responsible for retransmitting the packet.
Furthermore, any packets sent after the last good packet will also be resent since the
receiver has dropped any packets after the corrupt packet.
The transmitter keeps track of packets that have been sent but not acknowledged
through the use of a retry buffer. Transactions are added to the buffer as they are on
the PCI Express port. Transactions are removed from the buffer after they have been
acknowledged by the receiver.
Seq #
DLLP
TLP
CRC
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