Reads

0

0

A2 A1 A0

0

1

0

0

A2

A1 A0

0

1

S

0

A

A

A

R/W

A

P

NA

S

R/W

1

MSB

LSB

MSB

LSB

Slave Address

Acknowledge

From Slave

Command Byte

Data From Upper

or Lower Byte

of Register

Last Byte

Data

Acknowledge

From Slave

Acknowledge

From Slave

Slave Address

Data From Lower

or Upper Byte

of Register

First Byte

Data

No Acknowledge

From Master

Acknowledge

From Master

At this moment, master transmitter

becomes master receiver, and

slave-receiver becomes

slave-transmitter.

1

2

3

4

5

6

7

8

9

S

0

1

0

0

A2

A1

A0

1

A

7

6

5

4

3

2

1

0

A

I0.x

7

6

5

4

3

2

1

0

A

I1.x

7

6

5

4

3

2

1

0

A

I0.x

7

6

5

4

3

2

1

0

1

I1.x

P

R/W

SCL

SDA

INT

Read From Port 0

Data Into Port 0

Read From Port 1

Data Into Port 1

Acknowledge

From Master

Acknowledge

From Slave

Acknowledge

From Master

Acknowledge

From Master

No Acknowledge

From Master

PCA9555

The bus master first must send the PCA9555 address with the least-significant bit set to a logic 0 (see Figure 4

for device address). The command byte is sent after the address and determines which register is accessed.

After a restart, the device address is sent again, but this time, the least-significant bit is set to a logic 1. Data

from the register defined by the command byte then is sent by the PCA9555 (see Figure 8 through Figure 10).

After a restart, the value of the register defined by the command byte matches the register being accessed when

the restart occurred. For example, if the command byte references Input Port 1 before the restart, and the restart

occurs when Input Port 0 is being read, the stored command byte changes to reference Input Port 0. The original

command byte is forgotten. If a subsequent restart occurs, Input Port 0 is read first. Data is clocked into the

register on the rising edge of the ACK clock pulse. After the first byte is read, additional bytes may be read, but

the data now reflect the information in the other register in the pair. For example, if Input Port 1 is read, the next

byte read is Input Port 0.

Data is clocked into the register on the rising edge of the ACK clock pulse. There is no limitation on the number

of data bytes received in one read transmission, but when the final byte is received, the bus master must not

acknowledge the data.

Figure 8. Read From Register

A.

Transfer of data can be stopped at any time by a Stop condition. When this occurs, data present at the latest

acknowledge phase is valid (output mode). It is assumed that the command byte previously has been set to 00 (read

Input Port register).

B.

This figure eliminates the command byte transfer, a restart, and slave address call between the initial slave address

call and actual data transfer from the P port (see Figure 8 for these details).

Figure 9. Read Input Port Register, Scenario 1

Copyright © 2005–2008, Texas Instruments Incorporated

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