TCA8418E_15 Datasheet, PDF (24/50 Pages) Texas Instruments – I2C Controlled Keypad Scan IC With Integrated ESD Protection

TCA8418E
SCPS222C – MAY 2010 – REVISED OCTOBER 2015
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Programming (continued)
8.5.2.2 Reads
Reading from a slave is very similar to writing, but requires some additional steps. In order to read from a slave,
the master must first instruct the slave which register it wishes to read from. This is done by the master starting
off the transmission in a similar fashion as the write, by sending the address with the R/W bit equal to 0
(signifying a write), followed by the register address it wishes to read from. Once the slave acknowledges this
register address, the master will send a START condition again, followed by the slave address with the R/W bit
set to 1 (signifying a read). This time, the slave will acknowledge the read request, and the master will release
the SDA bus but will continue supplying the clock to the slave. During this part of the transaction, the master will
become the master-receiver, and the slave will become the slave-transmitter.
The master will continue to send out the clock pulses, but will release the SDA line so that the slave can transmit
data. At the end of every byte of data, the master will send an ACK to the slave, letting the slave know that it is
ready for more data. Once the master has received the number of bytes it is expecting, it will send a NACK,
signaling to the slave to halt communications and release the bus. The master will follow this up with a STOP
condition.
Figure 26 shows an example of reading a single byte from a slave register.
Master controls SDA line
Slave controls SDA line
Read from one register in a device
Device (Slave) Address (7 bits)
Register Address N (8 bits)
Device (Slave) Address (7 bits)
Data Byte from Register N (8 bits)
S 0 1 1 0 1 0 0 0 A B7 B6 B5 B4 B3 B2 B1 B0 A Sr 0 1 1 0 1 0 0 1 A D7 D6 D5 D4 D3 D2 D1 D0 NA P
START
R/W=0 ACK
ACK Repeated START
R/W=1 ACK
Figure 26. Read from Register
NACK STOP
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