Xilinx LogiCore PLB PCI Full Bridge - Manual (Page 34)

PLB PCI Full Bridge (v1.00a)
34 www.xilinx.com DS508 March 21, 2006
Product Specification
EARL
Y ACCESS
• Address translations in both directions are performed by high-order address bits substitution in the
address vector before crossing to the other bus domain. Byte addressing integrity is maintained
between buses.
• The user’s system must be designed to accomodate certain restrictions on throttling by the PLB PCI
Bridge. Both PLB and PCI burst transactions may be broken up into multiple transactions on the
target or slave bus due to restrictions on bus protocol and modules in the PLB PCI bridge.
Additional PLB and PCI transactions are automatically initiated when needed to complete a
transaction. The first restriction is that the v3.0 core does not permit throttling of data as either the
initiator or target except for insertion of wait states prior to the first data transfer. Another
restriction is, that as a master on the PLB, the PLB PCI Bridge is not allowed to throttle, but the PCI
remote initiator can cause the need to throttle on the PLB. This is particularly true when the PCI
clock is significantly slower than the PLB clock. The PLB PCI Bridge circumvents the throttling
limitations by terminating transactions as needed and reinitiating the request to continue as
needed. Parameters allow the user to optimize the burst size for high data throughput and
minimizing the number of transactions needed to complete the desired burst transactions.
• The interrupt status register in the IPIF contains information to identify an error conditions during
the implementation of the PLB PCI bridge and the troubleshooting of the system. To clear the
interrupt register bits that were "set" with an error condition, a write of a "1" to the bit position
corresponding to the operation must be performed.
• The v3.0 core does not permit throttling of data at either the initiator or target except for insertion of
wait states prior to the first data transfer. Consequently, if the PLB device requires throttling that
affects the PCI transaction, the PLB PCI Bridge must terminate the transaction. If the v3.0 core is the
initiator, a new PCI transaction must be initiated to continue data transfer. Although PLB masters
are not allowed to throttle data flow, the combined IPIF and PLB PCI Bridge operation can result in
the need for throttling data on the PCI bus, especially when the PLB clock is slower than the PCI
clock. The PLB PCI Bridge handles throttling by terminating initiator transactions as needed and
continuing the PLB master request with a new PCI transaction. Similarly, new PLB transactions are
automatically initiated when needed to complete a PCI initiator transaction.
PLB Master Initiates a Read Request of a PCI target
This section discusses the operation of a PLB master initiating single, burst and cacheline reads of a
remote PCI target. In these transactions, the v3.0 core is the PCI initiator.
The operation is similar whether the PCI space is memory or I/O space with the exception of the
command sent to the v3.0 core. A parameter associated with each BAR must be consistent with the
remote PCI device memory type as either I/O or memory. Based on this parameter setting, either I/O
or memory commands are asserted.
The PLB IPIF and bridge can accept both fixed length and arbitrary
length (i.e., burst length is determined by PLB_rdBurst signal) burst transactions on the PLB. Only one
PLB master read of a PCI target is supported at a time.
Commands supported in PLB master read operations are I/O read, memory read, and memory read
multiple. The command used is based on the address and qualifier decode, which includes the address,
memory type (i.e., I/O or memory type), and if burst is asserted.
Table 15 shows translations of PLB
transactions to PCI commands.
The address presented on the PLB is translated to the PCI address space by high-order bit substitution
with the 2 lsbs set as follows:
• If the target PCI address space is memory space, the 2 lsbs are set to 00 (i.e., linear incrementing