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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.

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.

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Altera PCI Compiler Manual do Utilizador Página 151