4
FN8209.2
August 31, 2010
PRINCIPLES OF OPERATION
SERIAL INTERFACE
Serial Interface Conventions
The device supports a bidirectional bus oriented protocol.
The protocol defines any device that sends data onto the
bus as a transmitter, and the receiving device as the
receiver. The device controlling the transfer is called the
master and the device being controlled is called the
slave. The master always initiates data transfers, and
provides the clock for both transmit and receive opera-
tions. Therefore, the X9523 operates as a slave in all
applications.
Serial Clock and Data
Data states on the SDA line can change only while SCL
is LOW. SDA state changes while SCL is HIGH are
reserved for indicating START and STOP conditions.
See Figure 1.On power-up of the X9523, the SDA pin is
in the input mode.
Serial Start Condition
All commands are preceded by the START condition,
which is a HIGH to LOW transition of SDA while SCL is
HIGH. The device continuously monitors the SDA and
SCL lines for the START condition and does not respond
to any command until this condition has been met. See
Figure 2.
Serial Stop Condition
All communications must be terminated by a STOP
condition, which is a LOW to HIGH transition of SDA
while SCL is HIGH. The STOP condition is also used to
place the device into the Standby power mode after a
read sequence. A STOP condition can only be issued
after the transmitting device has released the bus. See
Figure 2.
Serial Acknowledge
An ACKNOWLEDGE (ACK) is a software convention
used to indicate a successful data transfer. The trans-
mitting device, either master or slave, will release the
bus after transmitting eight bits. During the ninth clock
cycle, the receiver will pull the SDA line LOW to
ACKNOWLEDGE that it received the eight bits of data.
Refer to Figure 3.
The device will respond with an ACKNOWLEDGE after
recognition of a START condition if the correct Device
Identifier bits are contained in the Slave Address Byte. If
a write operation is selected, the device will respond with
an ACKNOWLEDGE after the receipt of each subse-
quent eight bit word.
In the read mode, the device will transmit eight bits of
data, release the SDA line, then monitor the line for an
ACKNOWLEDGE. If an ACKNOWLEDGE is detected
and no STOP condition is generated by the master, the
device will continue to transmit data. The device will ter-
SCL
SDA
Data Stable Data Change Data Stable
Figure 1. Valid Data Changes on the SDA Bus
SCL
SDA
Start Stop
Figure 2. Valid Start and Stop Conditions
X9523
