24LC21A

DS21160F-page 12

 2003 Microchip Technology Inc.

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 one. There are three basic types

of read operations: current address read, random read

and sequential read.

7.1

Current Address Read

The 24LC21A contains an address counter that

maintains the address of the last word accessed,

internally 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 one, the 24LC21A

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 24LC21A

discontinues transmission (Figure 7-1).

FIGURE 7-1:

CURRENT ADDRESS

READ

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

24LC21A 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 one. The 24LC21A will then

issue an acknowledge and transmits the 8-bit data

word. The master will not acknowledge the transfer but

does generate a Stop condition and the 24LC21A

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 24LC21A transmits

the first data byte, the master issues an acknowledge

as opposed to a Stop condition in a random read. This

directs the 24LC21A to transmit the next sequentially

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

To provide sequential reads the 24LC21A 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

is below 1.5 volts at nominal conditions.

The SDA, SCL and VCLK inputs have Schmitt Trigger

and filter circuits which suppress noise spikes to assure

proper device operation even on a noisy bus.

CONTROL

A

C

K

SP

BYTE

DATA n

BUS ACTIVITY

SDA LINE

BUS ACTIVITY

A

C

K

N

O

MASTER

101 0000 1

S

T

O

P

S

T

A

R

T