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Interrupt Output (INT)

Bus Transactions

Writes

SCL

Start Condition

Data 1 Valid

SDA

Write to Port

Data Out

From Port

R/W

ACK From Slave

ACK From Slave

ACK From Slave

1

9

8

7

6

5

4

3

2

Data 1

1

A2

0

1

S

1

1

A1 A0 0

A

0

0

0

0

0

0

0

A

A

P

Data to Port

Command Byte

Slave Address

SCL

SDA

Data to

Register

Start Condition

R/W

ACK From Slave

ACK From Slave

ACK From Slave

1

9

8

7

6

5

4

3

2

Data

1/0

A2

0

1

S

1

1

A1 A0 0

A

1

0

0

0

0

0

0

A

A

P

Data to Register

Command Byte

Slave Address

PCA9554A

WITH INTERRUPT OUTPUT AND CONFIGURATION REGISTERS

SCPS127A – SEPTEMBER 2006 – REVISED FEBRUARY 2007

signal INT is valid. Resetting the interrupt circuit is achieved when data on the port is changed to the original

setting; data is read from the port that generated the interrupt or in a Stop event. Resetting occurs in the read

mode at the acknowledge (ACK) bit or not acknowledge (NACK) bit after the rising edge of the SCL signal. In a

Stop event, INT is cleared after the rising edge of SDA. Interrupts that occur during the ACK or NACK clock

pulse can be lost (or be very short) due to the resetting of the interrupt during this pulse. Each change of the

I/Os after resetting is detected and is transmitted as INT.

Reading from or writing to another device does not affect the interrupt circuit, and a pin configured as an output

cannot cause an interrupt. Changing an I/O from an output to an input may cause a false interrupt to occur if the

state of the pin does not match the contents of the Input Port register.

Data is exchanged between the master and PCA9554A through write and read commands.

Data is transmitted to the PCA9554A by sending the device address and setting the least-significant bit to a

logic 0 (see Figure 4 for device address). The command byte is sent after the address and determines which

register receives the data that follows the command byte. There is no limitation on the number of data bytes sent

in one write transmission.

Figure 6. Write to Output Port Register

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Figure 7. Write to Configuration or Polarity Inversion Registers

9

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