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11

Reading a single byte from a slave: Unlike the write

operation that uses the specified memory address byte

to define where the data is to be written, the read opera-

tion occurs at the present value of the memory address

counter. To read a single byte from the slave, the master

generates a START condition, writes the slave address

byte with R/W = 1, reads the data byte with a NACK to

indicate the end of the transfer, and generates a STOP

condition. However, because requiring the master to

keep track of the memory address counter is impracti-

cal, the following method should be used to perform

reads from a specified memory location.

Manipulating the address counter for reads: A dummy

write cycle can be used to force the address counter to a

particular value. To do this the master generates a

START condition, writes the slave address byte (R/W =

0), writes the memory address where it desires to read,

generates a repeated START condition, writes the slave

address byte (R/W = 1), reads data with ACK or NACK

as applicable, and generates a STOP condition.

See Figure 3 for a read example using the repeated

START condition to specify the starting memory location.

Applications Information

Power-Supply Decoupling

To achieve the best results when using the DS3503,

wiper-bias voltage pin (V+) with a 0.01µF or 0.1µF

capacitor. Use a high-quality ceramic surface-mount

capacitor if possible. Surface-mount components mini-

mize lead inductance, which improves performance,

and ceramic capacitors tend to have adequate high-

frequency response for decoupling applications.

SDA and SCL Pullup Resistors

SDA is an I/O with an open-collector output that

requires a pullup resistor to realize high-logic levels. A

master using either an open-collector output with a

pullup resistor or a push-pull output driver must be

used for SCL. Pullup resistor values should be chosen

to ensure that the rise and fall times listed in the

Electrical Characteristics are within specification. A typ-

ical value for the pullup resistors is 4.7k

Ω.

SLAVE

ADDRESS

START

START

0

1

0

1

0

0

0

R/W

SLAVE

ACK

SLAVE

ACK

SLAVE

ACK

MSB

LSB

MSB

LSB

MSB

LSB

b7

b6

b5

b4

b3

b2

b1

b0

READ/

WRITE

REGISTER ADDRESS

b7

b6

b5

b4

b3

b2

b1

b0

DATA

STOP

SINGLE-BYTE WRITE

-WRITE STEP CONTROL

REGISTER (SCR) TO 1Fh

SINGLE-BYTE READ

-READ CONTROL REGISTER (CR)

START

REPEATED

START

51h

MASTER

NACK

STOP

01010000

00000 010

02h

01010 001

01010000

00000 001

00011111

50h

01h

1Fh

STOP

DATA

50h

A)

B)

SLAVE

ACK

SLAVE

ACK

SLAVE

ACK

SLAVE

ACK

SLAVE

ACK

SLAVE

ACK