Datasheet

Feature/Protocol Descriptions
30
March 5 2007 − June 2011SCPS154C
SCL From
Master
123 789
SDA Output
By Transmitter
SDA Output
By Receiver
Figure 3−9. 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 3.6.3,
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 3−10 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.
Sb6 b4b5 b3 b2 b1 b0 0 b7 b6 b5 b4 b3 b2 b1 b0AA
Slave Address Word Address
R/W
S/P = Start/Stop ConditionA = Slave Acknowledgement
b7 b6 b4b5 b3 b2 b1 b0 A P
Data Byte
Figure 3−10. Serial-Bus Protocol—Byte Write
Figure 3−11 illustrates a single byte read. The bridge issues a start condition and sends the 7-bit slave device
address and the R/W
command bit is equal to 0b (write). The slave device acknowledges if it recognizes the
slave address. Next, the EEPROM word address is sent by the bridge, and another slave acknowledgment
is expected. Then, the bridge issues a restart condition followed by the 7-bit slave address and the R/W
command bit is equal to 1b (read). Once again, the slave device responds with an acknowledge. Next, the
slave device sends the 8-bit data byte, MSB first. Since this is a 1-byte read, the bridge responds with no
acknowledge (logic high) indicating the last data byte. Finally, the bridge issues a stop condition.
Not Recommended for New Designs