© 1996 Microchip Technology Inc.

DS21095F-page 9

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 one. 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 main-

tains 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 24LC21 issues an acknowl-

edge and transmits the eight bit data word. The master

will not acknowledge the transfer but does generate a

stop condition and the 24LC21 discontinues transmis-

sion (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 one. The 24LC21 will then

issue 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 discon-

tinues transmission (Figure 7-2).

7.3

Sequential Read

Sequential reads are initiated in the same way as a ran-

dom 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

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

FIGURE 7-2:

RANDOM READ

SP

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

S

T

A

R

T

CONTROL

BYTE

DATA n

A

C

K

N

O

A

C

K

S

T

O

P

S

P

S

BUS ACTIVITY

MASTER

SDA LINE

BUS ACTIVITY

S

T

A

R

T

S

T

O

P

CONTROL

BYTE

WORD

ADDRESS (n)

DATA n

A

C

K

A

C

K

N

O

A

C

K

CONTROL

BYTE

A

C

K

S

T

A

R

T