Philips Semiconductors
Product data
PCA9554/PCA9554A
8-bit I2C and SMBus I/O port with interrupt
2002 Jul 26
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
bit
I7
I6
I5
I4
I3
I2
I1
I0
default
1
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 Register 3. Writes to this register have no effect.
Register 1 – Output Port Register
bit
O7
O6
O5
O4
O3
O2
O1
O0
default
1
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. Reads from this register return 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
0
This register allows the user to invert the polarity of the Input Port
Register data. If a bit in this register is set (written with ‘1’), the
corresponding Input Port data is inverted. If a bit in this register is
cleared (written with a ‘0’), the Input Port data polarity is retained.
Register 3 – Configuration Register
bit
C7
C6
C5
C4
C3
C2
C1
C0
default
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. At reset, the I/Os are
configured as inputs with a weak pull-up to VDD.
Power-on Reset
When power is applied to VDD, an internal power-on reset holds the
PCA9554 in a reset state until VDD has reached VPOR. At that point,
the reset condition is released and the PCA9554 registers and state
machine will initialize to their default states.
Interrupt Output
The open-drain interrupt output is activated when one of the port
pins change state and the pin is configured as an input. The
interrupt is deactivated when the input returns to its previous state or
the input port register is read.
Note that 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.