2. PCI Interface
35
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
2.1.4.1
Transactions Between Px and Py
PowerSpan II implements the following transaction ordering rules for transactions flowing between
PCI Interface Px and PCI Interface Py:
•
The order in which delayed read requests are latched on the source bus, and posted memory write
transactions are presented on the source bus, is the order in which they appear on the destination
bus.
•
Writes flowing from Px to Py have no ordering relationship to writes flowing from Py to Px.
•
The acceptance of a posted write as a target or slave is not contingent on the completion of a
transaction by the master of the same interface. PowerSpan II master and target/slave modules are
independent.
2.1.4.2
Transactions Between the PB Interface and the PCI Interfaces
When there are transactions to the PB Interface from both PCI-1 and PCI-2, there is a possibility that a
transaction from PCI-2 can be queued ahead of a transaction from
PCI-. This is caused by the fact there is no transaction ordering between the two independent PCI
interfaces. For example, if transactions to the PB Interface arrive in the following order from PCI-1 and
PCI-2:
•
PCI-1 Write 1
•
PCI-2 Write 1
•
PCI-2 Write 2
•
PCI-1 Write 2
The transactions can be completed to the PB Interface in the following order even though PCI-2 Write
2 entered PowerSpan II before PCI-1 Write 2:
•
PCI-1 Write 1
•
PCI-2 Write 1
•
PCI-1 Write 2
•
PCI-2 Write 2
This is caused by the fact that PCI-1 to PB Interface transactions and PCI-2 to PB Interface transactions
arbitrate in a round robin fashion. When a PowerSpan II decision is required on whether to service a
transaction from PCI-1 or PCI-2, writes are available at both even though at one point a write is only
available from PCI-2.
2.1.4.3
DMA Transactions
DMA transactions and regular write/read transactions arbitrate for the use of a master interface in a
round robin scheme. There are no special priorities for DMA transactions and regular write/read
transactions.