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master transmits (sends) data to the selected slave, and a one in this position means that the master
receives data from the slave.
Figure 18-5. R/S Bit in First Byte
R/S
LSB
Slave address
MSB
18.3.1.3
Data Validity
The data on the SDA line must be stable during the high period of the clock, and the data line can
only change when SCL is Low (see Figure 18-6).
Figure 18-6. Data Validity During Bit Transfer on the I
2
C Bus
Change
of data
allowed
Data line
stable
SDA
SCL
18.3.1.4
Acknowledge
All bus transactions have a required acknowledge clock cycle that is generated by the master. During
the acknowledge cycle, the transmitter (which can be the master or slave) releases the SDA line.
To acknowledge the transaction, the receiver must pull down SDA during the acknowledge clock
cycle. The data transmitted out by the receiver during the acknowledge cycle must comply with the
data validity requirements described in “Data Validity” on page 1280.
When a slave receiver does not acknowledge the slave address, SDA must be left High by the slave
so that the master can generate a STOP condition and abort the current transfer. If the master
device is acting as a receiver during a transfer, it is responsible for acknowledging each transfer
made by the slave. Because the master controls the number of bytes in the transfer, it signals the
end of data to the slave transmitter by not generating an acknowledge on the last data byte. The
slave transmitter must then release SDA to allow the master to generate the STOP or a repeated
START condition.
If the slave is required to provide a manual ACK or NACK, the
I
2
C Slave ACK Control
(I2CSACKCTL)
register allows the slave to NACK for invalid data or command or ACK for valid
data or command. When this operation is enabled, the MCU slave module I
2
C clock is pulled low
after the last data bit until this register is written with the indicated response.
18.3.1.5
Repeated Start
The I
2
C master module has the capability of executing a repeated START (transmit or receive) after
an initial transfer has occurred.
A repeated start sequence for a Master transmit is as follows:
1.
When the device is in the idle state, the Master writes the slave address to the
I2CMSA
register
and configures the
R/S
bit for the desired transfer type.
June 18, 2014
1280
Texas Instruments-Production Data
Inter-Integrated Circuit (I
2
C) Interface