10

S4242/S42WD42/S4261/S42WD61

2025 6.0 4/17/00

FIGURE 12. CURRENT ADDRESS BYTE READ MODE

FIGURE 11. ACKNOWLEDGE POLLING

Acknowledge Polling

When the S42xxx is performing an internal WRITE opera-

tion, it will ignore any new START conditions. Since the

device will only return an acknowledge after it accepts the

START, the part can be continuously queried until an

acknowledge is issued, indicating that the internal WRITE

cycle is complete.

To poll the device, give it a START condition, followed by

a slave address for a WRITE operation (See Figure 9).

READ OPERATIONS

Read operations are initiated with the R/W bit of the

identification field set to “1.” There are four different read

options:

1.

Current Address Byte Read

2.

Random Address Byte Read

3.

Current Address Sequential Read

4.

Random Address Sequential Read

Current Address Byte Read

The S42xxx contains an internal address counter which

maintains the address of the last word accessed,

incremented by one. If the last address accessed (either

a read or write) was to address location n, the next read

operation would access data from address location n+1

and increment the current address pointer. When the

S42xxx receives the slave address field with the R/W bit

set to “1,” it issues an acknowledge and transmits the 8-

bit word stored at address location n+1.

The current address byte read operation only accesses a

single byte of data. The master does not acknowledge the

transfer, but does generate a stop condition. At this point,

the S42xxx discontinues data transmission. See

Figure 12 for the address acknowledge and data transfer

sequence.

Issue Start

Internal WRITE Cycle

In Progress;

Begin ACK Polling

Issue Slave

Address and

R/W = 0

ACK

Returned?

Next

operation a

WRITE?

Issue Byte

Address

Proceed with

WRITE

Issue Stop

Await Next

Command

Issue Stop

No

No

Yes (Internal WRITE Cycle is completed)

Yes

2025 ILL11.0

S

T

A

R

T

S

T

O

P

Slave Address

Device

Type

Address

Read/Write

1= Read

A10,A9,A8

SDA Bus Activity

D

7

D

6

D

5

D

4

D

3

D

2

D

1

D

0

Master sends Read

request to Slave

Slave sends

Data to Master

Master Transmitter

to

Slave Receiver

Slave Transmitter

to

Master Receiver

1

1

1

00

1

Lack of ACK (low)

from Master

determines last

data byte to be read

1

Shading Denotes

42xxx

SDA Output Active

A

9

A

10

R

W

A

C

K

A

8

Data Byte

2025 ILL12.1