Altera PCI Compiler - Manual (Page 151)

Altera Corporation User Guide Version 11.1 3–77
October 2011
Functional Description
Target Transaction Terminations
For all transactions except configuration transactions, the local-side
device can request a transaction to be terminated with one of several
termination schemes defined by the PCI Local Bus Specification, Revision
3.0. The local-side device can use the lt_discn signal to request a retry
or disconnect. These termination types are considered graceful
terminations and are normally used by a target device to indicate that it
is not ready to receive or supply the requested data. A retry termination
forces the PCI master that initiated the transaction to retry the same
transaction at a later time. A disconnect, on the other hand, does not force
the PCI master to retry the same transaction.
The local-side device can also request a target abort, which indicates that
a catastrophic error has occurred in the device. This termination is
requested by asserting lt_abortn during a target transaction other than
a configuration transaction.
f
For more details on these termination types, refer to the PCI Local Bus
Specification, Revision 3.0.
Retry
The local-side device can request a retry if, for example, the device cannot
meet the initial latency requirement or because the local resource cannot
transfer data. A target device signals a retry by asserting devseln and
stopn
, while deasserting trdyn before the first data phase. The local-
side device can request a retry as long as it did not supply or request at
least one data phase in a burst transaction. In a write transaction, the
local-side device may request a retry by asserting lt_discn as long as it
did not assert the lt_rdyn signal to indicate it is ready for a data transfer.
If lt_rdyn is asserted, it can result in the PCI MegaCore function
asserting the trdyn signal on the PCI bus. Therefore, asserting
lt_discn
forces a disconnect instead of a retry. In a read transaction, the
local-side device can request a retry as long as data has not been
transferred to the PCI MegaCore function.
Figure 3–23 applies to all PCI
MegaCore functions, excluding the 64-bit signals as noted for pci_mt32
and pci_t32.