start

msb Chip Address lsb

w

ack

msb Register Add lsb

ack

msb

DATA

lsb

ack

stop

ack from slave

ack from slave

ack from slave

SCL

start

Id = 53h

w

ack

addr = 10h

ack

ack

data = 08h

stop

SDA

SCL

S

P

P

STO

condition

START condition

SDA

LM3553

SNVS414B – FEBRUARY 2008 – REVISED MAY 2013

www.ti.com

START AND STOP CONDITIONS

START and STOP conditions classify the beginning and the end of the I2C session. A START condition is

defined as SDA signal transitioning from HIGH to LOW while SCL line is HIGH. A STOP condition is defined as

the SDA transitioning from LOW to HIGH while SCL is HIGH. The I2C master always generates START and

STOP conditions. The I2C bus is considered to be busy after a START condition and free after a STOP condition.

During data transmission, the I2C master can generate repeated START conditions. First START and repeated

START conditions are equivalent, function-wise. The data on SDA line must be stable during the HIGH period of

the clock signal (SCL). In other words, the state of the data line can only be changed when CLK is LOW.

Figure 17. Start and Stop Conditions

TRANSFERRING DATA

Every byte put on the SDA line must be eight bits long, with the most significant bit (MSB) being transferred first.

Each byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated

by the master. The master releases the SDA line (HIGH) during the acknowledge clock pulse. The LM3553 pulls

down the SDA line during the 9th clock pulse, signifying an acknowledge. The LM3553 generates an

acknowledge after each byte has been received.

After the START condition, the I2C master sends a chip address. This address is seven bits long followed by an

eighth bit which is a data direction bit (R/W). The LM3553 address is 53h. For the eighth bit, a “0” indicates a

WRITE and a “1” indicates a READ. The second byte selects the register to which the data will be written. The

third byte contains data to write to the selected register.

w = write (SDA = "0")

ack = acknowledge (SDA pulled down by the slave)

id = chip address, 53h for LM3553

Figure 18. Write Cycle

I

2C COMPATIBLE CHIP ADDRESS

The chip address for LM3553 is 1010011, or 53hex.

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