LP3921 Datasheet, PDF (29/34 Pages) National Semiconductor (TI) – Battery Charger Management and Regulator Unit with Integrated Boomer® Audio Amplifier

I²C Compatible Serial Bus Interface
INTERFACE BUS OVERVIEW
The I²C compatible synchronous serial interface provides ac-
cess to the programmable functions and registers on the
device.
This protocol uses a two-wire interface for bi-directional com-
munications between the IC’s connected to the bus. The two
interface lines are the Serial Data Line (SDA), and the Serial
Clock Line (SCL). These lines should be connected to a pos-
itive supply, via a pull-up resistor of 1.5 kΩ, and remain HIGH
even when the bus is idle.
Every device on the bus is assigned a unique address and
acts as either a Master or a Slave depending on whether it
generates or receives the serial clock (SCL).
DATA TRANSACTIONS
One data bit is transferred during each clock pulse. Data is
sampled during the high state of the serial clock (SCL). Con-
sequently, throughout the clock’s high period, the data should
remain stable. Any changes on the SDA line during the high
state of the SCL and in the middle of a transaction, aborts the
current transaction. New data should be sent during the low
SCL state. This protocol permits a single data line to transfer
both command/control information and data using the syn-
chronous serial clock.
FIGURE 10. Bit Transfer
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Each data transaction is composed of a Start Condition, a
number of byte transfers (set by the software) and a Stop
Condition to terminate the transaction. Every byte written to
the SDA bus must be 8 bits long and is transferred with the
most significant bit first. After each byte, an Acknowledge sig-
nal must follow. The following sections provide further details
of this process.
START AND STOP
The Master device on the bus always generates the Start and
Stop Conditions (control codes). After a Start Condition is
generated, the bus is considered busy and it retains this sta-
tus until a certain time after a Stop Condition is generated. A
high-to-low transition of the data line (SDA) while the clock
(SCL) is high indicates a Start Condition. A low-to-high tran-
sition of the SDA line while the SCL is high indicates a Stop
Condition.
30069810
FIGURE 11. Start and Stop Conditions
In addition to the first Start Condition, a repeated Start Con-
dition can be generated in the middle of a transaction. This
allows another device to be accessed, or a register read cycle.
ACKNOWLEDGE CYCLE
The Acknowledge Cycle consists of two signals: the acknowl-
edge clock pulse the master sends with each byte transferred,
and the acknowledge signal sent by the receiving device.
The master generates the acknowledge clock pulse on the
ninth clock pulse of the byte transfer. The transmitter releases
the SDA line (permits it to go high) to allow the receiver to
send the acknowledge signal. The receiver must pull down
the SDA line during the acknowledge clock pulse and ensure
that SDA remains low during the high period of the clock
pulse, thus signaling the correct reception of the last data byte
and its readiness to receive the next byte.
FIGURE 12. Bus Acknowledge Cycle
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