SLLSEB6B – JULY 2012 – REVISED MAY 2013
The address and the 8-bit data bytes are sent most significant bit (MSB) first. The START bit is indicated by a
high-to-low transition of SDA while SCL is high. The STOP condition is created by a low-to-high transition of SDA
while SCL is high.
If the master writes to a slave, it repeatedly sends a byte with the slave sending an ACK bit. In this case, the
master is in master-transmit mode and the slave is in slave-receive mode.
If the master reads from a slave, it repeatedly receives a byte from the slave, while acknowledging (ACK) the
receipt of every byte but the last one (see
In this situation the master is in master-receive mode and
the slave is in slave-transmit mode.
The master ends the transmission with a STOP bit, or may send another START bit to maintain bus control for
further transfers.
S Slave Address
W
A
From Master to Slave
From Slave to Master
S Slave Address R A
DATA
A
DATA
AP
DATA
A
DATA
AP
A = acknowledge
A = not acknowledge
S = Start
P = Stop
R = Read
W = Write
Master Transmitter writing to Slave Receiver
Master Receiver reading from Slave Transmitter
Figure 8. Transmit or Receive Mode Changes During a Data Transfer
When writing to a slave, a master mainly operates in transmit-mode and only changes to receive-mode when
receiving acknowledgment from the slave.
When reading from a slave, the master starts in transmit-mode and then changes to receive-mode after sending
a READ request (R/W bit = 1) to the slave. The slave continues in the complementary mode until the end of a
transaction.
Note, that the master ends a reading sequence by not acknowledging (NACK) the last byte received. This
procedure resets the slave state machine and allows the master to send the STOP command.
Isolator Functional Principle
To isolate a bidirectional signal path (SDA or SCL), the ISO1540 internally splits a bidirectional line into two
unidirectional signal lines, each of which is isolated via a single-channel digital isolator. Each channel output is
made open-drain to comply with the open-drain technology of I
2
C. Side 1 of the ISO1540 connects to a low-
capacitance I
2
C node, while Side 2 is designed for connecting to a fully loaded I
2
C bus with up to 400 pF
capacitance.
VCC1
A
R
PU1
SDA1
ISO1540
C
node
C
GND1
V
REF
D
GND2
V
ILT1
V
IHT1
V
OL1
C
bus
40mV
50mV
V
SDA1
B
R
PU2
SDA2
VCC2
V
C-out
Figure 9. SDA Channel Design and Voltage Levels at SDA1
At first sight, the arrangement of the internal buffers suggests a closed signal loop that is prone to latch-up.
However, this loop is broken by implementing an output buffer (B) whose output low-level is raised by a diode
drop to approximately 0.75 V, and the input buffer (C) that consists of a comparator with defined hysteresis. The
comparator’s upper and lower input thresholds then distinguish between the proper low-potential of 0.4 V
maximum driven directly by SDA1 and the buffered output low-level of B.
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