PCI7421 datasheet(68/298 Pages) TI

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3−12

SDA

SCL

Start

Condition

Stop

Condition

Change of

Data Allowed

Data Line Stable,

Data Valid

Figure 3−7. Serial-Bus Start/Stop Conditions and Bit Transfers

Data is transferred serially in 8-bit bytes. The number of bytes that may be transmitted during a data transfer is

unlimited; however, each byte must be completed with an acknowledge bit. An acknowledge (ACK) is indicated by

the receiver pulling the SDA signal low, so that it remains low during the high state of the SCL signal. Figure 3−8

illustrates the acknowledge protocol.

SCL From

Master

123

SDA Output

By Transmitter

SDA Output

By Receiver

Figure 3−8. Serial-Bus Protocol Acknowledge

The PCI7x21/PCI7x11 controller is a serial bus master; all other devices connected to the serial bus external to the

PCI7x21/PCI7x11 controller are slave devices. As the bus master, the PCI7x21/PCI7x11 controller drives the SCL

clock at nearly 100 kHz during bus cycles and places SCL in a high-impedance state (zero frequency) during idle

states.

Typically, the PCI7x21/PCI7x11 controller masters byte reads and byte writes under software control. Doubleword

reads are performed by the serial EEPROM initialization circuitry upon a PCI reset and may not be generated under

software control. See Section 3.6.4, Serial-Bus EEPROM Application, for details on how the PCI7x21/PCI7x11

controller automatically loads the subsystem identification and other register defaults through a serial-bus EEPROM.

Figure 3−9 illustrates a byte write. The PCI7x21/PCI7x11 controller issues a start condition and sends the 7-bit slave

device address and the command bit zero. A 0 in the R/W command bit indicates that the data transfer is a write. The

slave device acknowledges if it recognizes the address. If no acknowledgment is received by the PCI7x21/PCI7x11

controller, then an appropriate status bit is set in the serial-bus control/status register (PCI offset B3h, see

Section 4.50). The word address byte is then sent by the PCI7x21/PCI7x11 controller, and another slave

acknowledgment is expected. Then the PCI7x21/PCI7x11 controller 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 3−9. Serial-Bus Protocol—Byte Write

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