X9521 datasheet(3/24 Pages) RENESAS

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X9521

FN8207 Rev 2.00

Page 3 of 24

September 21, 2010

PRINCIPLES OF OPERATION

SERIAL INTERFACE

Serial Interface Conventions

The device supports a bidirectional bus oriented protocol.

The protocol defines any device that sends data onto the

bus as a transmitter, and the receiving device as the

receiver. The device controlling the transfer is called the

master and the device being controlled is called the slave.

The master always initiates data transfers, and provides

the clock for both transmit and receive operations. There-

fore, the X9521 operates as a slave in all applications.

Serial Clock and Data

Data states on the SDA line can change only while SCL is

LOW. SDA state changes while SCL is HIGH are reserved

for indicating START and STOP conditions. See Figure 1.

On power-up of the X9521, the SDA pin is in the input

mode.

Serial Start Condition

All commands are preceded by the START condition,

which is a HIGH to LOW transition of SDA while SCL is

HIGH. The device continuously monitors the SDA and SCL

lines for the START condition and does not respond to any

command until this condition has been met. See Figure 2.

Serial Stop Condition

All communications must be terminated by a STOP condi-

tion, which is a LOW to HIGH transition of SDA while SCL

is HIGH. The STOP condition is also used to place the

device into the Standby power mode after a read

sequence. A STOP condition can only be issued after the

transmitting device has released the bus. See Figure 2.

Serial Acknowledge

An ACKNOWLEDGE (ACK) is a software convention used

to indicate a successful data transfer. The transmitting

device, either master or slave, will release the bus after

transmitting eight bits. During the ninth clock cycle, the

receiver will pull the SDA line LOW to ACKNOWLEDGE

that it received the eight bits of data. Refer to Figure 3.

The device will respond with an ACKNOWLEDGE after

recognition of a START condition if the correct Device Iden-

tifier bits are contained in the Slave Address Byte. If a write

operation is selected, the device will respond with an

ACKNOWLEDGE after the receipt of each subsequent

eight bit word.

In the read mode, the device will transmit eight bits of data,

release the SDA line, then monitor the line for an

ACKNOWLEDGE. If an ACKNOWLEDGE is detected and

no STOP condition is generated by the master, the device

will continue to transmit data. The device will terminate fur-

ther data transmissions if an ACKNOWLEDGE is not

detected. The master must then issue a STOP condition to

place the device into a known state.

SCL

SDA

Data Stable

Data Change

Data Stable

Figure 1.

Valid Data Changes on the SDA Bus

SCL

SDA

Start

Stop

Figure 2.

Valid Start and Stop Conditions

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