LPS225HB datasheet(23/47 Pages) STMICROELECTRONICS

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LPS225HB

Digital interfaces

6.2.1

I2C operation

The transaction on the bus is started through a START (ST) signal. A start condition is

defined as a HIGH-to-LOW transition on the data line while the SCL line is held HIGH. After

this has been transmitted by the master, the bus is considered busy. The next data byte

transmitted after the start condition contains the address of the slave in the first 7 bits and

the eighth bit tells whether the master is receiving data from the slave or transmitting data to

the slave. When an address is sent, each device in the system compares the first seven bits

after a start condition with its address. If they match, the device considers itself addressed by

the master.

The slave address (SAD) associated to the LPS225HB is 101110xb. The SDO/SA0 pad can

be used to modify the less significant bit of the device address. If the SA0 pad is connected

to voltage supply, LSb is ‘1’ (address 1011101b), otherwise if the SA0 pad is connected to

ground, the LSb value is ‘0’ (address 1011100b). This solution permits to connect and

address two different LPS225HB devices to the same I²C lines.

Data transfer with acknowledge is mandatory. The transmitter must release the SDA line

during the acknowledge pulse. The receiver must then pull the data line LOW so that it

remains stable low during the HIGH period of the acknowledge clock pulse. A receiver which

has been addressed is obliged to generate an acknowledge after each byte of data received.

The I²C embedded in the LPS225HB behaves like a slave device and the following protocol

must be adhered to. After the start condition (ST) a slave address is sent, once a slave

acknowledge (SAK) has been returned, an 8-bit sub-address (SUB) will be transmitted: the 7

LSB represents the actual register address while the MSB enables address auto increment.

If the MSb of the SUB field is ‘1’, the SUB (register address) will be automatically increased

to allow multiple data read/write.

The slave address is completed with a Read/Write bit. If the bit is ‘1’ (Read), a repeated

START (SR) condition must be issued after the two sub-address bytes; if the bit is ‘0’ (Write)

the master will transmit to the slave with direction unchanged. Table 10 explains how the

SAD+read/write bit pattern is composed, listing all the possible configurations.

Table 10. SAD+Read/Write patterns

Command

SAD[6:1]

SAD[0] = SA0

R/W

SAD+R/W

Read

101110

10111001 (B9h)

Write

101110

10111000 (B8h)

Read

101110

10111011 (BBh)

Write

101110

10111010 (BAh)

Table 11. Transfer when master is writing one byte to slave

Master

SAD + W

SUB

DATA

Slave

SAK

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