PLB PCI Full Bridge (v1.00a)
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DS508 March 21, 2006
Product Specification
EARL
Y ACCESS
when an incomplete PCI transactions occur or when PCI errors occur. Details of the abnormal
terminations are discussed in a later section. In these transactions, the v3.0 core is the PCI initiator.
The operation is essentially the same whether the PCI space is memory or I/O space; the only
difference is the command sent to the v3.0 core by the PLB PCI Bridge. The bridge can accept both fixed
length and arbitrary length burst transactions on the PLB. All PLB burst transfers are 64-bits per data
phase; dynamic byte enable is not supported by the PLB protocol. The length of a burst defined as
arbitrary length is defined by the master signal
PLB_wrBurst
. The PLB specification requires all
cacheline write transactions to be sequential fill type, independent of the target word; however, the PLB
IPIF requires the address received during a cacheline write operation to be the first word of the line
being written.
Commands supported in PLB master write operations are I/O write and memory write (both single
and burst). The command used is based on the address/qualifier decode, which includes the address,
memory type (i.e., I/O or memory type), if a double word is written and if
PLB_wrBurst
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 mode). If the PCI target address space is IO-space, the 2 LSBs are passed
unchanged from that presented on the PLB bus.
Both single and burst write transfers are posted so the data is buffered in the IPIF2PCI FIFO, which has
a depth defined by the parameter C_IPIF2PCI_FIFO_ABUS_WIDTH. Due to the FIFO backup
requirement of the v3.0 core, the FIFO usable buffer depth is the actual depth minus 3 words.
Data is loaded in the FIFO on each clock cycle that the write request is asserted and the address decode
is valid. If the transaction is not a burst (i.e., PLB_wrBurst is not high), two cases can occur because the
PLB bus is 64-bit and the PCI bus is 32-bit. If the PLB transfer is a single word or bytes within a single
word, a single PCI transaction (I/O or Memory Write command) is performed. If the PLB transfer is a
double word or bytes within both words of the double word, a burst of 2 words is performed on the PCI
bus. In PLB burst transfers (i.e., PLB_wrBurst is asserted), the data is buffered and the PCI transfer is
initiated when the FIFO is filled to the level defined by the parameter
C_TRIG_PCI_DATA_XFER_OCC_LEVEL or when the PLB write is completed.
Only one PLB master write to a PCI target is supported at a time. Write transactions are not queued in
the bridge. After the PLB write to the bridge is completed and while a write to PCI is being completed,
the PLB PCI Bridge asserts PLB rearbitrate to terminate subsequent PLB transactions. When a posted
write is complete, another write request from a PLB master can be initiated.
Consistent with the PCI specification, the PLB PCI Bridge re-issues commands when an PCI retry is
asserted. To avoid permanent livelock, the posted write is attempted to be completed up to a
predefined number of retries defined by the parameter C_NUM_PCI_RETRIES_IN_WRITES.
Re-issuing the write operation on the PCI is automatic.
It is the responsibility of the master to properly write data to a PCI target from non-prefetchable PLB
sources. For example, it must perform single transaction reads of non-prefetchable PLB sources to
avoid loss of data in fire-and-forget writes to a PCI target.
In addition, the user must insure that any burst writes do not attempt to write beyond a valid address
range. The PLB IPIF does not check for valid address during data phases. Therefoe, during a burst, it
will accept data that is correlated to an address beyond the current range. The PLB PCI Bridge will
transfer the data on the PCI if it is received without error flagging. It is the user’s responsibility not to
