CAT5409

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7

SERIAL BUS PROTOCOL

1. Data transfer may be initiated only when the bus is

not busy.

2. During a data transfer, the data line must remain

stable whenever the clock line is high. Any

changes in the data line while the clock is high

will be interpreted as a START or STOP condition.

The device controlling the transfer is a master, typically a

processor or controller, and the device being controlled is the

slave. The master will always initiate data transfers and

provide the clock for both transmit and receive operations.

Therefore, the CAT5409 will be considered a slave device

in all applications.

START Condition

The START Condition precedes all commands to the

device, and is defined as a HIGH to LOW transition of SDA

when SCL is HIGH. The CAT5409 monitors the SDA and

SCL lines and will not respond until this condition is met.

STOP Condition

A LOW to HIGH transition of SDA when SCL is HIGH

determines the STOP condition. All operations must end

with a STOP condition.

DEVICE ADDRESSING

The bus Master begins a transmission by sending a

START condition. The Master then sends the address of the

particular slave device it is requesting. The four most

significant bits of the 8−bit slave address are fixed as 0101

for the CAT5409 (see Figure 6). The next four significant

bits (A3, A2, A1, A0) are the device address bits and define

which device the Master is accessing. Up to sixteen devices

may be individually addressed by the system. Typically,

+5 V and ground are hard-wired to these pins to establish the

device’s address.

After the Master sends a START condition and the slave

address byte, the CAT5409 monitors the bus and responds

with an acknowledge (on the SDA line) when its address

matches the transmitted slave address.

Acknowledge

After a successful data transfer, each receiving device is

required to generate an acknowledge. The Acknowledging

device pulls down the SDA line during the ninth clock cycle,

signaling that it received the 8 bits of data.

The CAT5409 responds with an acknowledge after

receiving a START condition and its slave address. If the

device has been selected along with a write operation, it

responds with an acknowledge after receiving each 8-bit

byte.

When the CAT5409 is in a READ mode it transmits 8 bits

of data, releases the SDA line, and monitors the line for an

acknowledge. Once it receives this acknowledge, the

CAT5409 will continue to transmit data. If no acknowledge

is sent by the Master, the device terminates data transmission

and waits for a STOP condition.

Figure 5. Acknowledge Timing

ACKNOWLEDGE

1

START

SCL FROM

MASTER

89

DATA OUTPUT

FROM TRANSMITTER

DATA OUTPUT

FROM RECEIVER