L3GD20H datasheet(28/52 Pages) STMICROELECTRONICS

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Digital interfaces

L3GD20H

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DocID023469 Rev 2

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 L3GD20H is 110101xb. SDO/SA0 pin can be

used to modify less significant bit of the device address. If SDO/SA0 pin is connected to

voltage supply LSb is ‘1’ (address 1101011b) else if SDO/SA0 pin is connected to ground

LSb value is ‘0’ (address 1101010b). This solution permits to connect and address two

different gyroscopes to the same I

C bus.

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 I

C embedded inside the L3GD20H 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 will be 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) will be 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 will transmit to the slave with direction unchanged.

Table 12 explains how

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

Table 12. SAD+Read/Write patterns

Command

SAD[6:1]

SAD[0] = SDO

R/W

SAD+R/W

Read

110101

11010101 (D5h)

Write

110101

11010100 (D4h)

Read

110101

11010111 (D7h)

Write

110101

11010110 (D6h)

Table 13. Transfer when Master is writing one byte to slave

Master

SAD + W

SUB

DATA

Slave

SAK

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