LSM320HAY30 datasheet(21/42 Pages) STMICROELECTRONICS

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LSM320HAY30

Digital interfaces

Doc ID 16917 Rev 1

21/42

6.1.1

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 byte of data

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

the 8th 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 with the LSM320HAY30 is 001100xb. The SDO/SA0

pad can be used to modify the least significant bit of the device address. If the SA0 pad is

connected to voltage supply, LSb is ‘1’ (address 0011001b), otherwise if the SA0 pad is

connected to ground, the LSb value is ‘0’ (address 0011000b). This solution permits

connecting and addressing two different accelerometers 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 LSM320HAY30 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) is transmitted: the

7 LSb represent the actual register address while the MSB enables address auto-increment.

If the MSb of the SUB field is ‘1’, the SUB (register address) is automatically incremented to

allow multiple data read/write.

The slave address is completed with a read/write bit. If the bit was ‘1’ (read), a repeated

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

(write) the master transmits to the slave with direction unchanged.

Table 9 explains how the

SAD+Read/Write bit pattern is composed, listing all the possible configurations.

Table 9.

SAD+Read/Write patterns

Command

SAD[6:1]

SAD[0] = SA0

R/W

SAD+R/W

Read

001100

00110001 (31h)

Write

001100

00110000 (30h)

Read

001100

00110011 (33h)

Write

001100

00110010 (32h)

Table 10.

Transfer when master is writing one byte to slave

Master

SAD + W

SUB

DATA

Slave

SAK

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