Altera HyperTransport MegaCore Function - Manual (Page 50)

3–24 Chapter 3: Specifications
HyperTransport MegaCore Function Specification
HyperTransport MegaCore Function User Guide © November 2009 Altera Corporation
Preliminary
Tx Buffer Ordering
Packets in the Tx buffers are transmitted in the same order they are written to the
different buffers. For example, the response buffer transmits all packets in the order
they are written into it. This implementation satisfies all ordering rule requirements in
the HT specification except for response and non-posted packets that have the
PassPW bit cleared. If a response or non-posted packet has the PassPW bit cleared, the
HT specification requires that the response or non-posted packet must not be
transmitted before the preceding posted packet.
DatEna_i Input Data transfer enable. This signal functions as the ena signal in the Atlantic interface
specification with the HyperTransport MegaCore function as a slave sink. DatEna_i is
driven by the interface master and used to control the flow of data across the interface.
DatEna_i behaves as a write enable.
Dav_o Output Data buffer is available. This signal functions as dav in the Atlantic interface. When Dav_o
is high, it indicates that the buffer has sufficient storage for at least one complete packet
including command and data.
If there is room for at least two complete, full-size packets (9 64-bit words per packet)
when a start of packet is written,
Dav_o remains asserted during the packet so that
another packet can be written immediately after the first. However, if there is only room for
one full-size packet,
Dav_o is deasserted after the start of the packet is written and
reasserted again when the buffer has the room for at least one additional full-size packet.
The local-side application must sample Dav_o asserted to write the start of the packet, but
it should ignore Dav_o to write the subsequent words of the packet.
When a single word packet (i.e. with Sop_i and Eop_i asserted in the same cycle) is
written, it is typically not possible to start another packet in the following cycle. If there is
initially only room for one additional packet, then
Dav_o is deasserted in the cycle
following the single word packet making it illegal to start writing a packet in that cycle.
Typically the user application must write the single word packet in cycle one, sample
Dav_o in cycle two and if Dav_o is still asserted, start writing a new packet in cycle 3.
Special note for TxRDav_o only: Because the Tx response buffer also handles responses
generated internally in the HyperTransport MegaCore function by the CSR and end-chain
modules, the
TxRDav_o signal can be deasserted at any time due to the internal usage. By
default, the HyperTransport MegaCore function accepts response packets written by the
user interface for one cycle after
TxRDav_o is deasserted. If the local-side application
requires multiple clock cycles between sampling TxRDav_o asserted and writing the start
of packet (for example, due to a complex state machine or data pipeline), the window for
accepting packets can be increased by changing the
TxRDavToSopDelay configuration
parameter.
TxPDav_o and TxNpDav_o are only deasserted in response to user-written packets or
reset.
Table 3–7. Tx Command/Data Buffer Interface Signals (Part 2 of 2)
Signal Name Direction Description