S4261PAS datasheet(8/16 Pages) SUMMIT

8 / 16 page

S4242/S42WD42/S4261/S42WD61

2025 6.0 4/17/00

FIGURE 8. ACKNOWLEDGE RESPONSE FROM RECEIVER

CHARACTERISTICS OF THE I2C BUS

General Description

The I2C bus was designed for two-way, two-line serial

communication between different integrated circuits. The

two lines are: a serial data line (SDA), and a serial clock

line (SCL). The SDA line must be connected to a positive

supply by a pull-up resistor, located somewhere on the

bus (See Figure 6). Data transfer between devices may

be initiated with a START condition only when SCL and

SDA are HIGH (bus is not busy).

Input Data Protocol

One data bit is transferred during each clock pulse. The

data on the SDA line must remain stable during clock

HIGH time, because changes on the data line while SCL

is HIGH will be interpreted as start or stop condition, refer

to Figure 2.

START and STOP Conditions

When both the data and clock lines are HIGH, the bus is

said to be not busy. A HIGH-to-LOW transition on the data

line, while the clock is HIGH, is defined as the “START”

condition. A LOW-to-HIGH transition on the data line,

while the clock is HIGH, is defined as the “STOP” condi-

tion (See Figure 2).

DEVICE OPERATION

The S42xxx is a 16K-bit serial E2PROM. The device

supports the I2C bidirectional data transmission protocol.

The protocol defines any device that sends data onto the

bus as a “transmitter” and any device which receives data

as a “receiver.” The device controlling data transmission

is called the “master” and the controlled device is called

the “slave.” In all cases, the S42xxx will be a “slave”

device, since it never initiates any data transfers.

Acknowledge (ACK)

Acknowledge is a software convention used to indicate

successful data transfers. The transmitting device, either

the master or the slave, will release the bus after transmit-

ting eight bits. During the ninth clock cycle, the receiver

FIGURE 9. SLAVE ADDRESS BYTE

will pull the SDA line LOW to ACKnowledge that it received

the eight bits of data (See Figure 8).

The S42xxx will respond with an ACKnowledge after

recognition of a START condition and its slave address

byte. If both the device and a write operation are selected,

the S42xxx will respond with an ACKnowledge after the

receipt of each subsequent 8-bit word.

In the READ mode, the S42xxx transmits eight bits of

data, then releases the SDA line, and monitors the line for

an ACKnowledge signal. If an ACKnowledge is detected,

and no STOP condition is generated by the master, the

S42xxx will continue to transmit data. If an ACKnowledge

is not detected, the S42xxx will terminate further data

transmissions and awaits a STOP condition before return-

ing to the standby power mode.

Device Addressing

Following a start condition the master must output the

address of the slave it is accessing. The most significant

four bits of the slave address are the device type identifier

(see figure 7). For the S42xxx this is fixed as 1010[B].

Word Address

The next three bits of the slave address are an extension

of the array’s address and are concatenated with the eight

bits of address in the word address field, providing direct

access to the 2,048 x8 array of the S4261 and S42WD61.

A10 and A9 are “Don’t Care” on S4242 and S42WD42.

Read/Write Bit

The last bit of the data stream defines the operation to be

performed. When set to “1,” a read operation is selected;

when set to “0,” a write operation is selected.

SCL from

Master

Data Output

from

Transmitter

Data Output

from

Receiver

Start

Condition

ACKnowledge

tAA

2025 ILL8.0

A10

R/W

*S4261/S42WD61 only

DEVICE

IDENTIFIER

HIGH ORDER

WORD ADDRESS

2025 ILL9.1

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