XIO2000AI TAOS [TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS], XIO2000AI Datasheet - Page 51

• Manufacturer Part Number: XIO2000AI
• Description: PCI Express to PCI Bus Translation Bridge
• Manufacturer: TAOS [TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS]

3.10 Two-Wire Serial-Bus Interface
April 2007 Revised October 2008
Because PCI Express does not have a unique locked-memory write request packet, all PCI Express memory
write requests that are received while the bridge is locked are considered part of the locked sequence and
are transmitted to PCI as locked-memory write transactions. In addition, all traffic mapped to VC1 is allowed
to pass.
The bridge terminates the locked sequence when an unlock message is received from PCI Express and all
previous locked transactions have been completed.
In the erroneous case that a normal downstream memory read request is received during a locked sequence,
the bridge responds with an unsupported request completion status. Please note that this condition must
never occur, because the PCI Express specification requires the root complex to block normal memory read
requests at the source. All locked sequences that end successfully or with an error condition must be
immediately followed by an unlock message. This unlock message is required to return the bridge to a known
unlocked state.
The bridge provides a two-wire serial-bus interface to load subsystem identification information and specific
register defaults from an external EEPROM. The serial-bus interface signals (SCL and SDA) are shared with
two of the GPIO terminals (4 and 5). If the serial bus interface is enabled, then the GPIO4 and GPIO5 terminals
are disabled. If the serial bus interface is disabled, then the GPIO terminals operate as described in
Section 3.13.
DEVSEL
FRAME
FRAME
LOCK
LOCK
TRDY
IRDY
IRDY
CLK
CLK
AD
Figure 3−13. Terminating A Locked Sequence
Figure 3−12. Continuing A Locked Sequence
Address
Data
Feature/Protocol Descriptions
SCPS155C
41