Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
255
Address Translation Unit (PCI Express)—Intel
®
81341 and 81342
3.3.3
Outbound Write Transaction
An outbound write transaction is initiated by the Intel XScale
®
processor
7
or by one of
the DMAs and is targeted at a PCI Express domain. The outbound write address and
write data are propagated from the 81341 and 81342 internal bus to a PCI Express Link
through the OPHQ and OPDQ, respectively.
The ATUs internal bus target interface claims the write transaction and forwards it to
the PCI Express Link. The data flow for an outbound write transaction on the internal
bus is summarized in the following statements:
• ATU internal bus target interface latches the address from the internal bus into the
OPHQ when that address is inside one of the outbound translate windows (see
) and the OPHQ is not full.
• When the OPHQ is full, the target interface signals a Retry on the internal bus to
the outbound cycle initiator.
• Once outbound address is latched, internal bus target interface stores write data
into the OPDQ until the internal bus transaction completes or the reaches a buffer
boundary.
• When the data is latched in a buffer in OPDQ, the outbound cycle is enabled for
transmission on the PCI Express Link.
The PCI interface is responsible for completing the outbound write transaction with the
PCI address translated from the OPHQ and the data in the OPDQ. The data flow for an
outbound write transaction on the PCI Express Link is summarized in the following
statements:
• Writes transactions is fragmented based on the Max_Payload_Size parameter. A
write issues when the Max_Payload_Size is reached, or the write disconnects on
the internal bus.
• When Posted Header and Posted Data credits are available a memory write request
TLP is issued on the PCI Express Link.
• When a data parity error is detected while pulling data from the OPDQ, the TLP is
poisoned.
7. For best performance, the user should designate the two Outbound Memory Windows as non-
cachable and bufferable from the Intel XScale
®
processore. This assignment enables the Intel
XScale
®
processor to issue multiple outstanding transactions to the Outbound Memory Windows,
thereby, taking full advantage of the ATU outbound queue architecture. However, the user needs
to be aware that the Outbound ATU queue architecture does not maintain strict ordering between
read and write requests as described in
Table 126, “ATU Outbound Data Flow Ordering Rules” on
. In the event that the user requires strict ordering to be maintained, the user must
change the designation of this region of memory to be non-cachable/non-bufferable and enforce
the requirement in software.