2
I C bus specification
STA335BW
2
4
I C bus specification
2
The STA335BW supports the I C protocol via the input ports SCL and SDA_IN (Master to
Slave) and the output port SDA_OUT (Slave to Master). This protocol defines any device
that sends data on to the bus as a transmitter and any device that reads the data as a
receiver. The device that controls the data transfer is known as the master and the other as
the slave. The master always starts the transfer and provides the serial clock for
synchronization. STA335BW is always a slave device in all of its communications. It
2
supports up to 400 kb/sec rate (fast-mode bit rate). STA335BW I C is a slave only interface.
4.1
Communication protocol
4.1.1
Data transition or change
Data changes on the SDA line must only occur when the SCL clock is low. SDA transition
while the clock is high is used to identify a START or STOP condition.
4.1.2
4.1.3
4.1.4
Start condition
START is identified by a high to low transition of the data bus SDA signal while the clock
signal SCL is stable in the high state. A START condition must precede any command for
data transfer.
Stop condition
STOP is identified by low to high transition of the data bus SDA signal while the clock signal
SCL is stable in the high state. A STOP condition terminates communication between
STA335BW and the bus master.
Data input
During the data input the STA335BW samples the SDA signal on the rising edge of clock
SCL. For correct device operation the SDA signal must be stable during the rising edge of
the clock and the data can change only when the SCL line is low.
4.2
Device addressing
To start communication between the master and the STA335BW, the master must initiate
with a start condition. Following this, the master sends onto the SDA line 8-bits (MSB first)
corresponding to the device select address and read or write mode.
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The seven most significant bits are the device address identifiers, corresponding to the I C
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bus definition. In the STA335BW the I C interface has two device addresses depending on
the SA port configuration, 0x38 when SA = 0, and 0x3A when SA = 1.
The eighth bit (LSB) identifies read or write operation RW, this bit is set to 1 in read mode
and 0 for write mode. After a START condition the STA335BW identifies on the bus the
device address and if a match is found, it acknowledges the identification on SDA bus during
the 9th bit time. The byte following the device identification byte is the internal space
address.
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STMICROELECTRONICS [ ST ]
