Altera IP Compiler for PCI Express - Manual (Page 71)

Chapter 4: IP Core Architecture 4–13
Data Link Layer
August 2014 Altera Corporation IP Compiler for PCI Express User Guide
The data link layer has the following subblocks:
■ Data Link Control and Management State Machine—This state machine is
synchronized with the physical layer’s LTSSM state machine and is also connected
to the configuration space registers. It initializes the link and virtual channel flow
control credits and reports status to the configuration space. (Virtual channel 0 is
initialized by default, as is a second virtual channel if it has been physically
enabled and the software permits it.)
■ Power Management—This function handles the handshake to enter low power
mode. Such a transition is based on register values in the configuration space and
received PM DLLPs.
■ Data Link Layer Packet Generator and Checker—This block is associated with the
data link layer packet’s 16-bit CRC and maintains the integrity of transmitted
packets.
■ Transaction Layer Packet Generator—This block generates transmit packets,
generating a sequence number and a 32-bit CRC. The packets are also sent to the
retry buffer for internal storage. In retry mode, the transaction layer packet
generator receives the packets from the retry buffer and generates the CRC for the
transmit packet.
■ Retry Buffer—The retry buffer stores transaction layer packets and retransmits all
unacknowledged packets in the case of NAK DLLP reception. For ACK DLLP
reception, the retry buffer discards all acknowledged packets.
■ ACK/NAK Packets—The ACK/NAK block handles ACK/NAK data link layer
packets and generates the sequence number of transmitted packets.
■ Transaction Layer Packet Checker—This block checks the integrity of the received
transaction layer packet and generates a request for transmission of an ACK/NAK
data link layer packet.
■ TX Arbitration—This block arbitrates transactions, basing priority on the
following order:
1. Initialize FC data link layer packet
2. ACK/NAK data link layer packet (high priority)
3. Update FC data link layer packet (high priority)
4. PM data link layer packet
5. Retry buffer transaction layer packet
6. Transaction layer packet
7. Update FC data link layer packet (low priority)
8. ACK/NAK FC data link layer packet (low priority)