Philips Semiconductors

Product data

PCA9556

2002 Mar 28

4

REGISTERS

Command Byte

Command

Protocol

Function

0

Read byte

Input port register

1

Read/write byte

Output port register

2

Read/write byte

Polarity inversion register

3

Read/write byte

Configuration register

The command byte is the first byte to follow the address byte during

a write transmission. It is used as a pointer to determine which of the

following registers will be written or read.

Register 0 — Input Port Register

I7

I6

I5

I4

I3

I2

I1

I0

This register is an 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 register 3. Writes to this register have no effect.

Register 1 — Output Port Register

bit

O7

O6

O5

O4

O3

O2

O1

O0

default

0

0

0

0

0

0

0

0

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

outputs by register 3. 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.

Register 2 — Polarity Inversion Register

bit

N7

N6

N5

N4

N3

N2

N1

N0

default

1

1

1

1

0

0

0

0

This register enables polarity inversion of pins defined as inputs by

register 3. If a bit in this register is set (written with ‘1’), the

corresponding port pin’s polarity is inverted. If a bit in this register is

cleared (written with a ‘0’), the corresponding port pin’s original

polarity is retained.

Register 3 — Configuration Register

bit

C7

C6

C5

C4

C3

C2

C1

C0

default

1

1

1

1

1

1

1

1

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.

RESET

Power-on Reset

the reset condition is released and the PCA9556 registers and

SMBus state machine will initialize to their default states.

External Reset

A reset can be accomplished by holding the RESET pin low for a

machine will be held in their default state until the RESET input is