PCA9555
REMOTE 16-BIT I2C AND SMBus I/O EXPANDER
WITH INTERRUPT OUTPUT AND CONFIGURATION REGISTERS
www.ti.com
SCPS131D–AUGUST 2005–REVISED OCTOBER 2006
I2C Interface
The bidirectional I2C bus consists of the serial clock (SCL) and serial data (SDA) lines. Both lines must be
connected to a positive supply via a pullup resistor when connected to the output stages of a device. Data
transfer may be initiated only when the bus is not busy.
I2C communication with this device is initiated by a master sending a Start condition, a high-to-low transition on
the SDA input/output while the SCL input is high (see Figure 1). After the Start condition, the device address
byte is sent, MSB first, including the data direction bit (R/W). This device does not respond to the general call
address.
After receiving the valid address byte, this device responds with an ACK, a low on the SDA input/output during
the high of the ACK-related clock pulse. The address inputs (A0–A2) of the slave device must not be changed
between the Start and Stop conditions.
On the I2C bus, only one data bit is transferred during each clock pulse. The data on the SDA line must remain
stable during the high pulse of the clock period, as changes in the data line at this time are interpreted as control
commands (Start or Stop) (see Figure 2).
A Stop condition, a low-to-high transition on the SDA input/output while the SCL input is high, is sent by the
master (see Figure 1).
Any number of data bytes can be transferred from the transmitter to the receiver between the Start and the Stop
conditions. Each byte of eight bits is followed by one ACK bit. The transmitter must release the SDA line before
the receiver can send an ACK bit. The device that acknowledges must pull down the SDA line during the ACK
clock pulse so that the SDA line is stable low during the high pulse of the ACK-related clock period (see
Figure 3). When a slave receiver is addressed, it must generate an ACK after each byte is received. Similarly,
the master must generate an ACK after each byte that it receives from the slave transmitter. Setup and hold
times must be met to ensure proper operation.
A master receiver signals an end of data to the slave transmitter by not generating an acknowledge (NACK)
after the last byte has been clocked out of the slave. This is done by the master receiver by holding the SDA line
high. In this event, the transmitter must release the data line to enable the master to generate a Stop condition.
SDA
SCL
S
P
Start Condition
Stop Condition
Figure 1. Definition of Start and Stop Conditions
SDA
SCL
Data Line
Stable;
Data Valid
Change
of Data
Allowed
Figure 2. Bit Transfer
6
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