XIO2000 datasheet(46/150 Pages) TI

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Feature/Protocol Descriptions

November 2005

SCPS097B

SDA

SCL

Start

Condition

Stop

Condition

Change of

Data Allowed

Data Line Stable,

Data Valid

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

123

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.

Sb6

b3 b2 b1 b0

b7 b6 b5 b4 b3

b2 b1 b0

Slave Address

Word Address

R/W

S/P = Start/Stop Condition

A = Slave Acknowledgement

b7 b6

b3 b2 b1 b0

Data Byte

Figure 3−17. Serial-Bus Protocol—Byte Write

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