24LC21 datasheet(11/24 Pages) MICROCHIP

11 / 24 page

 2004 Microchip Technology Inc.

DS21095J-page 11

24LC21

7.0

READ OPERATION

Read operations are initiated in the same way as write

operations with the exception that the R/W bit of the

slave address is set to ‘1’. There are three basic types

of read operations: current address read, random read

and sequential read.

7.1

Current Address Read

The 24LC21 contains an address counter that

maintains the address of the last word accessed, inter-

nally incremented by one. Therefore, if the previous

access (either a read or write operation) was to

address n, the next current address read operation

would access data from address n + 1. Upon receipt of

the slave address with R/W bit set to ‘1’, the 24LC21

issues an acknowledge and transmits the eight bit data

word. The master will not acknowledge the transfer but

does generate a Stop condition and the 24LC21

discontinues transmission (Figure 7-1).

7.2

Random Read

Random read operations allow the master to access

any memory location in a random manner. To perform

this type of read operation, first the word address must

be set. This is done by sending the word address to the

24LC21 as part of a write operation. After the word

address is sent, the master generates a Start condition

following the acknowledge. This terminates the write

operation, but not before the internal address pointer is

set. Then the master issues the control byte again but

with the R/W bit set to a ‘1’. The 24LC21 will then issue

an acknowledge and transmits the eight bit data word.

The master will not acknowledge the transfer but does

generate

Stop

condition

and

the

24LC21

discontinues transmission (Figure 7-2).

7.3

Sequential Read

Sequential reads are initiated in the same way as a

random read except that after the 24LC21 transmits the

first data byte, the master issues an acknowledge as

opposed to a Stop condition in a random read. This

directs the 24LC21 to transmit the next sequentially

addressed 8-bit word (see Figure 7-3).

To provide sequential reads the 24LC21 contains an

internal address pointer which is incremented by one at

the completion of each operation. This address pointer

allows the entire memory contents to be serially read

during one operation.

7.4

Noise Protection

The 24LC21 employs a VCC threshold detector circuit

which disables the internal erase/write logic if the VCC

is below 1.5 volts at nominal conditions.

The SCL and SDA inputs have Schmitt Trigger and

filter circuits which suppress noise spikes to assure

proper device operation even on a noisy bus.

FIGURE 7-1:

CURRENT ADDRESS READ

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

Control

Byte

Data (n)

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