PI7C21P100B
2-PORT PCI-X TO PCI-X BRIDGE
4.4.3
TYPE 1 TO TYPE 1 FORWARDING
Type 1 to Type 1 transaction forwarding provides a hierarchical configuration mechanism
when two or more levels of PCI-to-PCI bridges are used.
When PI7C21P100B detects a Type 1 configuration transaction intended for a PCI/PCI-X bus
downstream from the secondary interface, PI7C21P100B forwards the transaction unchanged
to the secondary interface. Ultimately, this transaction is translated to a Type 0 configuration
command or to a special cycle transaction by a downstream PCI bridge. Downstream Type 1
to Type 1 forwarding occurs when the following conditions are met during the address phase:
The lowest two address bits on P_AD[1:0] are equal to 01b.
The bus number falls in the range defined by the lower limit (exclusive) in the secondary
bus number register and the upper limit (inclusive) in the subordinate bus number
register.
P_AD[1:0] is a configuration read or configuration write transaction.
PI7C21P100B also supports Type 1 to Type 1 forwarding of configuration write transactions
upstream to support upstream special cycle generation. All upstream Type 1 configuration
read commands are ignored by PI7C21P100B.
PI7C21P100B forwards Type 1 to Type 1 configuration read and write transactions as delayed
transactions in the PCI mode and as split transactions in PCI-X mode.
4.4.4
SPECIAL CYCLES
The Type 1 configuration mechanism is used to generate special cycle transactions in
hierarchical PCI/PCI-X systems. Special cycle transactions can be generated from Type 1
configuration write transactions in either the upstream or the downstream direction.
PI7C21P100B initiates a special cycle on the target bus when a Type 1 configuration write
transaction is detected on the initiating bus and the following conditions are met during the
address phase:
The lowest two address bits on AD[1:0] are equal to 01b.
The device number in address bits AD[15:11] is equal to 11111b.
The function number in address bits AD[10:8] is equal to 111b.
The register number in address bits AD[7:2] is equal to 000000b.
The bus number is equal to the value in the secondary bus number register for
downstream transactions or equal to the value in the primary bus number register for
upstream transactions.
The bus command on CBE is a configuration write command.
When PI7C21P100B initiates the transaction on the target interface, the bus command is
changed from configuration write to special cycle. Devices that use special cycles ignore the
address and decode only the bus command. The data phase contains the special cycle
message. The transaction is forwarded as a delayed transaction in PCI mode and as a split
transaction in PCI-X mode. Once the transaction is completed on the target bus through
detection of the master abort condition, PI7C21P100B completes the transaction on the
initiating bus by accepting the retry on the delayed command in PCI mode or by generating a
Page 32 of 79
November 2005 – Revision 1.02
PERICOM [ PERICOM SEMICONDUCTOR CORPORATION ]
