© 2007 Microchip Technology Inc.

DS21189L-page 5

24AA64/24LC64/24FC64

2.0

FUNCTIONAL DESCRIPTION

The 24XX64 supports a bidirectional, 2-wire bus and

data transmission protocol. A device that sends data

onto the bus is defined as transmitter, while a device

receiving data is defined as a receiver. The bus has to

be controlled by a master device which generates the

Serial Clock (SCL), controls the bus access and

generates the Start and Stop conditions, while the

24XX64 works as slave. Both master and slave can

operate as transmitter or receiver, but the master

device determines which mode is activated.

3.0

BUS CHARACTERISTICS

The following bus protocol has been defined:

• Data transfer may be initiated only when the bus

is not busy

• During data transfer, the data line must remain

stable whenever the clock line is high. Changes in

the data line while the clock line is high will be

interpreted as a Start or Stop condition

Accordingly, the following bus conditions have been

defined (Figure 3-1).

3.1

Bus Not Busy (A)

Both data and clock lines remain high.

3.2

Start Data Transfer (B)

A high-to-low transition of the SDA line while the clock

(SCL) is high determines a Start condition. All

commands must be preceded by a Start condition.

3.3

Stop Data Transfer (C)

A low-to-high transition of the SDA line while the clock

(SCL) is high determines a Stop condition. All

operations must be ended with a Stop condition.

3.4

Data Valid (D)

The state of the data line represents valid data when,

after a Start condition, the data line is stable for the

duration of the high period of the clock signal.

The data on the line must be changed during the low

period of the clock signal. There is one clock pulse per

bit of data.

Each data transfer is initiated with a Start condition and

terminated with a Stop condition. The number of data

bytes transferred between Start and Stop conditions is

determined by the master device and is, theoretically,

unlimited (although only the last thirty two will be stored

when doing a write operation). When an overwrite does

occur, it will replace data in a first-in first-out (FIFO)

fashion.

3.5

Acknowledge

Each receiving device, when addressed, is obliged to

generate an acknowledge after the reception of each

byte. The master device must generate an extra clock

pulse which is associated with this Acknowledge bit.

The device that acknowledges has to pull down the

SDA line during the Acknowledge clock pulse in such a

way that the SDA line is stable low during the high

period of the acknowledge related clock pulse. Of

course, setup and hold times must be taken into

account. During reads, a master must signal an end of

data to the slave by not generating an Acknowledge bit

on the last byte that has been clocked out of the slave.

In this case, the slave (24XX64) will leave the data line

high to enable the master to generate the Stop

condition.

FIGURE 3-1:

DATA TRANSFER SEQUENCE ON THE SERIAL BUS

Note:

The 24XX64 does not generate any

Acknowledge

bits

if

an

internal

programming cycle is in progress.

SCL

SDA

(A)

(B)

(D)

(D)

(A)

(C)

Start

Condition

Address or

Acknowledge

Valid

Data

Allowed

to Change

Stop

Condition