LIS2DH datasheet(24/50 Pages) STMICROELECTRONICS

24 / 50 page

Digital interfaces

LIS2DH

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Doc ID 022516 Rev 1

5.1.1

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 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 LIS2DH is 001100xb. SDO/SA0 pad can be

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

supply, LSb is ‘1’ (address 0011001b) else if SA0 pad is connected to ground, LSb value is

‘0’ (address 0011000b). This solution permits to connect and address 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 inside the LIS2DH 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, a 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 increased 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 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 explains how the

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

Table 14.

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 15.

Transfer when master is writing one byte to slave

Master

SAD + W

SUB

DATA

Slave

SAK

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