24LCS52

DS21166B-page 4

Preliminary

© 1996 Microchip Technology Inc.

2.0

FUNCTIONAL DESCRIPTION

The 24LCS52 supports a bi-directional 2-wire bus and

data transmission protocol. A device that sends data

onto the bus is defined as transmitter, and a device

receiving data as 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 24LCS52

works as slave. Both master and slave can operate as

transmitter or receiver but the master device deter-

mines 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

the data bytes transferred between the START and

STOP conditions is determined by the master device

and is theoretically unlimited, although only the last six-

teen will be stored when doing a write operation. When

an overwrite does occur it will replace data in a first in

first out 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. 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 must leave the data line HIGH to enable the

master to generate the STOP condition.

3.6

Device Addressing

A control byte is the first byte received following the

START condition from the master device. The first part

of the control byte consists of a 4-bit control code which

is set to 1010 for normal read and write operations and

0110 for writing to the write protect register. The control

byte is followed by three chip select bits (A2, A1, A0).

The chip select bits allow the use of up to eight

24LCS52 devices on the same bus and are used to

determine which device is accessed. The chip select

bits in the control byte must correspond to the logic lev-

els on the corresponding A2, A1 and A0 pins for the

device to respond. The device will not acknowledge if

you attempt a read command with the control code set

to 0110.

Note:

The 24LCS52 does not generate any

acknowledge bits if an internal program-

ming cycle is in progress.

FIGURE 3-1:

DATA TRANSFER SEQUENCE ON THE SERIAL BUS CHARACTERISTICS

(A)

(B)

(C)

(D)

(A)

(C)

SCL

SDA

START

CONDITION

ADDRESS OR

ACKNOWLEDGE

VALID

DATA

ALLOWED

TO CHANGE

STOP

CONDITION