XIO2000A_08 Datasheet, PDF (53/159 Pages) TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS – PCI Express to PCI Bus Translation Bridge

Feature/Protocol Descriptions
SDA
SCL
Start
Condition
Stop
Condition
Data Line Stable,
Data Valid
Change of
Data Allowed
Figure 3−15. Serial-Bus Start/Stop Conditions and Bit Transfers
Data is transferred serially in 8-bit bytes. During a data transfer operation, the exact number of bytes that are
transmitted is unlimited. However, each byte must be followed by an acknowledge bit to continue the data
transfer operation. An acknowledge (ACK) is indicated by the data byte receiver pulling the SDA signal low,
so that it remains low during the high state of the SCL signal. Figure 3−16 illustrates the acknowledge protocol.
SCL From
Master
1
2
3
7
8
9
SDA Output
By Transmitter
SDA Output
By Receiver
Figure 3−16. Serial-Bus Protocol Acknowledge
The bridge performs three basic serial-bus operations: single byte reads, single byte writes, and multibyte
reads. The single byte operations occur under software control. The multibyte read operations are performed
by the serial EEPROM initialization circuitry immediately after a PCI Express reset. See Section 3.10.3,
Serial-Bus EEPROM Application, for details on how the bridge automatically loads the subsystem
identification and other register defaults from the serial-bus EEPROM.
Figure 3−17 illustrates a single byte write. The bridge issues a start condition and sends the 7-bit slave device
address and the R/W command bit is equal to 0b. A 0b in the R/W command bit indicates that the data transfer
is a write. The slave device acknowledges if it recognizes the slave address. If no acknowledgment is received
by the bridge, then bit 1 (SB_ERR) is set in the serial-bus control and status register (PCI offset B3h, see
Section 4.58). Next, the EEPROM word address is sent by the bridge, and another slave acknowledgment
is expected. Then the bridge delivers the data byte MSB first and expects a final acknowledgment before
issuing the stop condition.
Slave Address
Word Address
Data Byte
S b6 b5 b4 b3 b2 b1 b0 0 A b7 b6 b5 b4 b3 b2 b1 b0 A b7 b6 b5 b4 b3 b2 b1 b0 A P
R/W
A = Slave Acknowledgement
S/P = Start/Stop Condition
Figure 3−17. Serial-Bus Protocol—Byte Write
April 2007 Revised October 2008
SCPS155C
43