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Chapter 13. Input/Output Page 13-13

Figure 13-7. Addressing an I²C Peripheral

Each peripheral (slave) device on the I²C bus has a unique address. The first byte of an I²C trans-

action contains the address of the desired peripheral. Figure 13-7 shows the format for this first

byte, which is sometimes called a

control

byte.

The FX2 sends the bit sequence shown in Figure 13-7 to select the peripheral at a particular

address, to establish the transfer direction (using R/W

), and to determine if the peripheral is

present by testing for ACK

The four most significant bits (SA3:0) are the peripheral chip’s slave address. I²C devices are pre-

assigned slave addresses by device type. Slave address 1010, for example, is assigned to

EEPROMs. The next three bits (DA2:0) usually reflect the states of the peripheral’s device address

pins. Devices with three address pins can be strapped to allow eight distinct addresses for the

same device type, which allows, for example, up to eight identical serial EEPROMs to be individu-

ally addressed.

The eighth bit (R/W

) sets the direction for the ensuing data transfer (1 = master read, 0 = master

write). Most address transfers are followed by one or more data transfers, with the STOP condition

generated after the last data byte is transferred.

In Figure 13-7, a READ transfer follows the address byte (at clock 8, the master sets the R/W

bit

high, indicating READ). At clock 9, the peripheral device responds to its address by asserting ACK

At clock 10, the master floats SDA and issues SCL pulses to clock in SDA data supplied by the

slave.

13.4.2 Registers

The three registers shown in Figur e13-8 are used to conduct transfers over the I²C-compatible

bus.

Data is transferred to and from the bus through the I2DAT register. The I2CS register controls the

transfers and reports various status conditions. I2CTL configures the bus.

123456

8 9

SA3 ACKSA2 SA1 SA0 DA2 DA1 DA0

start

SDA

D7 D6

10 11

R/W

SCL

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Cypress Semiconductor FX2LP Technical Information Seite 287