Register Descriptions

Power-On Reset

RESET

PCA9557

The input port register (register 0) 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. It only acts on read operation. Writes to these

registers have no effect. The default value, X, is determined by the externally applied logic level.

input port register will be accessed next.

Register 0 (Input Port Register)

BIT

I7

I6

I5

I4

I3

I2

I1

I0

DEFAULT

X

X

X

X

X

X

X

X

The output port register (register 1) shows 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.

Register 1 (Output Port Register)

BIT

O7

O6

O5

O4

O3

O2

O1

O0

DEFAULT

0

0

0

0

0

0

0

0

The polarity inversion register (register 2) allows polarity inversion of pins defined as inputs by the configuration

register. 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 2 (Polarity Inversion Register)

BIT

N7

N6

N5

N4

N3

N2

N1

N0

DEFAULT

1

1

1

1

0

0

0

0

The configuration register (register 3) configures the directions of the I/O pins. If a bit in this register is set to 1,

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

cleared to 0, the corresponding port pin is enabled as an output.

Register 3 (Configuration Register)

BIT

C7

C6

C5

C4

C3

C2

C1

C0

DEFAULT

1

1

1

1

1

1

1

1

operating voltage for a power-reset cycle. The RESET input can be asserted to reset the system, while keeping

8

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