XIO2001IPNP datasheet(37/145 Pages) TI2

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SCL From

Master

SDA Output

By Transmitter

SDA Output

By Receiver

Figure 8-11. 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 8.3.8.3, Serial-Bus

EEPROM Application, for details on how the bridge automatically loads the subsystem identification and other

Figure 8-12 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 8.4.59). 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.

Figure 8-12. Serial-Bus Protocol – Byte Write

Figure 8-13 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.

Figure 8-13. Serial-Bus Protocol – Byte Read

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XIO2001

SCPS212J – MAY 2009 – REVISED JANUARY 2021

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