and Lookup Table

____________________________________________________________________

11

Slave devices: Slave devices send and receive data at

the master’s request.

Bus idle or not busy: Time between STOP and START

conditions when both SDA and SCL are inactive and in

their logic-high states.

START condition: A START condition is generated by

the master to initiate a new data transfer with a slave.

Transitioning SDA from high to low while SCL remains

high generates a START condition.

STOP condition: A STOP condition is generated by the

master to end a data transfer with a slave. Transitioning

SDA from low to high while SCL remains high generates

a STOP condition.

Repeated START condition: The master can use a

repeated START condition at the end of one data transfer

to indicate that it will immediately initiate a new data

transfer following the current one. Repeated starts are

commonly used during read operations to identify a spe-

cific memory address to begin a data transfer. A repeat-

ed START condition is issued identically to a normal

START condition.

Bit write: Transitions of SDA must occur during the low

state of SCL. The data on SDA must remain valid and

unchanged during the entire high pulse of SCL plus the

setup and hold time requirements. Data is shifted into the

device during the rising edge of the SCL.

Bit read: At the end of a write operation, the master

must release the SDA bus line for the proper amount of

setup time before the next rising edge of SCL during a

bit read. The device shifts out each bit of data on SDA

at the falling edge of the previous SCL pulse and the

data bit is valid at the rising edge of the current SCL

pulse. Remember that the master generates all SCL

clock pulses, including when it is reading bits from the

slave.

Acknowledge (ACK and NACK): An Acknowledge

(ACK) or Not Acknowledge (NACK) is always the 9th bit

transmitted during a byte transfer. The device receiving

data (the master during a read or the slave during a

write operation) performs an ACK by transmitting a 0

during the 9th bit. A device performs a NACK by trans-

mitting a 1 during the 9th bit. Timing for the ACK and

NACK is identical to all other bit writes. An ACK is the

acknowledgment that the device is properly receiving

data. A NACK is used to terminate a read sequence or

indicates that the device is not receiving data.

Byte write: A byte write consists of 8 bits of information

transferred from the master to the slave (most signifi-

cant bit first) plus a 1-bit acknowledgment from the

slave to the master. The 8 bits transmitted by the mas-

ter are done according to the bit write definition and the

acknowledgment is read using the bit read definition.

Byte read: A byte read is an 8-bit information transfer

from the slave to the master plus a 1-bit ACK or NACK

from the master to the slave. The 8 bits of information

that are transferred (most significant bit first) from the

slave to the master are read by the master using the bit

read definition above, and the master transmits an ACK

using the bit write definition to receive additional data

bytes. The master must NACK the last byte read to ter-

SDA

SCL

REPEATED

START

STOP

START

Figure 3. I2C Timing Diagram