A8285SLB datasheet(8/17 Pages) ALLEGRO

8 / 17 page

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A8285/A8287

LNB Supply and Control Voltage Regulator

Timing Considerations

The control sequence of the communication through the I2C

interface is composed of several steps in sequence:

1. Start Condition. Deﬁned by a negative edge on the SDA

line, while SCL is high.

2. Address Cycle. 7 bits of address, plus 1 bit to indicate

read (1) or write (0), and an acknowledge bit. The ﬁrst

ﬁve bits of the address are ﬁxed as: 00010. The four

optional addresses, deﬁned by the remaining two bits, are

selected by the ADD input. The address is transmitted

MSB ﬁrst.

3. Data Cycles. 8 bits of data followed by an acknowledge

bit. Multiple data bytes can be read. Data is transmitted

MSB ﬁrst.

4. Stop Condition. Deﬁned by a positive edge on the SDA

line, while SCL is high.

Except to indicate a Start or Stop condition, SDA must be

stable while the clock is high. SDA can only be changed

while SCL is low. It is possible for the Start or Stop condition

to occur at any time during a data transfer. The A8285/A8287

always responds by resetting the data transfer sequence.

The Read/Write bit is used to determine the data transfer

direction. If the Read/Write bit is high, the master reads one

or more bytes from the A8285/A8287. If the Read/Write bit

is low, the master writes one byte to the A8285/A8287. Note

that multiple writes are not permitted. All write operations

must be preceded with the address.

The Acknowledge bit has two functions. It is used by the

master to determine if the slave device is responding to its

address and data, and it is used by the slave when the master

is reading data back from the slave. When the A8285/A8287

decodes the 7-bit address ﬁeld as a valid address, it responds

by pulling SDA low during the ninth clock cycle.

During a data write from the master, the A8285/A8287 also

pulls SDA low during the clock cycle that follows the data

byte, in order to indicate that the data has been successfully

received. In both cases, the master device must release the

Control Data

Address

Start

Stop

SDA

SCL

acknowledge

from LNBR

acknowledge

from LNBR

NAK

Status Data

Address

Start

Stop

SDA

SCL

no acknowledge

from master

acknowledge

from LNBR

NAK

Status Data

Address

Start

Stop

Status Data

SDA

SCL

acknowledge

from master

acknowledge

from LNBR

no acknowledge

from master

Writing to the Register

Reading One Byte from the Register

Reading Multiple Bytes from the Register

Application Information

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