 2004 Microchip Technology Inc.

DS21140F-page 11

24LC41

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. These operations are identical for

both the DDC Monitor Port (in Bidirectional mode) and

the Microcontroller Access Port and are completely

independent of one another.

7.1

Current Address Read

The port contains an address counter that maintains

the address of the last word accessed, internally incre-

mented 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 port issues an acknowledge

and transmits the 8-bit data word. The master will not

acknowledge the transfer but does generate a Stop

condition and the port 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

port 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. The

master then issues the control byte again, but with

the R/W bit set to a one. The port then issues an

acknowledge and transmits the 8-bit data word. The

master will not acknowledge the transfer but does

generate a Stop condition and the port 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 port transmits the

first data byte, and the master issues an acknowledge

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

directs the port to transmit the next sequentially

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

To provide sequential reads, the port 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

Both the DDC Monitor Port and Microcontroller Access

below 1.5 volts at nominal conditions.

The DSCL, MSCL, DSDA and MSDA 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

SP

Bus Activity

Master

DSDA or

BUS Activity

S

T

A

R

T

Control

Byte

Data n

A

C

K

N

O

A

C

K

S

T

O

P

MSDA Line