STLED325 datasheet(31/62 Pages) STMICROELECTRONICS

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STLED325

Functional description

Doc ID 17576 Rev 1

31/62

3.18

Bus characteristics

The bus is intended for communication between different ICs. It consists of two lines: a bi-

directional data signal (SDA) and a clock signal (SCL). Both the SDA and SCL lines must be

connected to a positive supply voltage (typical voltage is 3.3 V) via a pull-up resistor (typical

value is 10 K). The following protocol has been defined.

- Data transfer may be initiated only when the bus is not busy.

- During data transfer, the data line must remain stable whenever the clock line is High.

- Changes in the data line, while the clock line is High, will be interpreted as control

signals.

Accordingly, the following bus conditions have been defined:

Bus not busy: Both data and clock lines remain High.

Start data transfer: A change in the state of the data line, from high to Low, while the clock

is High, defines the START condition.

Stop data transfer: A change in the state of the data line, from Low to High, while the clock

is High, defines the STOP condition.

Data valid: The state of the data line represents valid data when after a start condition, the

data line is stable for the duration of the high period of the clock signal. The data on the line

may be changed during the Low period of the clock signal. There is one clock pulse per bit

of data.

Each data transfer is initiated with a start condition and terminated with a stop condition.

The number of data bytes transferred between the start and stop conditions is not limited.

The information is transmitted byte-wide and each receiver acknowledges with a ninth bit.

By definition a device that gives out a message is called “transmitter,” the receiving device

that gets the message is called “receiver.” The device that controls the message is called

“master.” The devices that are controlled by the master are called “slaves.”

Acknowledge: Each byte of eight bits is followed by one Acknowledge Bit. This

Acknowledge Bit is a low level put on the bus by the receiver whereas the master generates

an extra acknowledge related clock pulse. A slave receiver which is addressed is obliged to

generate an acknowledge after the reception of each byte that has been clocked out of the

master transmitter.

The device that acknowledges has to pull down the SDA line during the acknowledge clock

pulse in such a way that the SDA line is a stable Low during the High period of the

acknowledge related clock pulse. Of course, setup and hold times must be taken into

account. A master receiver must signal an end of data to the slave transmitter by not

generating an acknowledge on the last byte that has been clocked out of the slave. In this

case the transmitter must leave the data line High to enable the master to generate the

STOP condition.

Note:

Refer to Philips I2C specification or contact STMicroelectronics for more information on I2C.

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