PCA9557

© NXP B.V. 2008. All rights reserved.

Product data sheet

Rev. 06 — 11 June 2008

8 of 26

NXP Semiconductors

PCA9557

7.3 Register descriptions

7.3.1 Register 0 - Input port register

This register is a read-only port. It reflects the incoming logic levels of the pins, regardless

of whether the pin is defined as an input or an output by the Configuration register. Writes

to this register have no effect.

7.3.2 Register 1 - Output port register

This register reflects the outgoing logic levels of the pins defined as outputs by the

Configuration register. Bit values in this register have no effect on pins defined as inputs.

In turn, reads from this register reflect the value that is in the flip-flop controlling the output

selection, not the actual pin value.

7.3.3 Register 2 - Polarity inversion register

This register enables polarity inversion of pins defined as inputs by the Configuration

register. If a bit in this register is set (written with logic 1), the corresponding port pin’s

polarity is inverted. If a bit in this register is cleared (written with logic 0), the

corresponding port pin’s original polarity is retained.

7.3.4 Register 3 - Configuration register

This register configures the directions of the I/O pins. If a bit in this register is set, the

corresponding port pin is enabled as an input with high-impedance output driver. If a bit in

this register is cleared, the corresponding port pin is enabled as an output.

Table 5.

Register 0 - Input port register bit allocation

Bit

7

6

5

4

3

2

1

0

Symbol

I7

I6

I5

I4

I3

I2

I1

I0

Table 6.

Register 1 - Output port register bit allocation

Bit

7

6

5

4

3

2

1

0

Symbol

O7

O6

O5

O4

O3

O2

O1

O0

Default

00000000

Table 7.

Register 2 - Polarity inversion register bit allocation

Bit

7

6

5

4

3

2

1

0

Symbol

N7

N6

N5

N4

N3

N2

N1

N0

Default

11110000

Table 8.

Register 3 - Configuration register bit allocation

Bit

7

6

5

4

3

2

1

0

Symbol

C7

C6

C5

C4

C3

C2

C1

C0

Default

11111111