The PCA9555 is a 24-pin CMOS device that provide 16 bits of
General Purpose parallel Input/Output (GPIO) expansion for
I2C/SMBus applications and was developed to enhance the Philips
family of I??C I/O expanders. The improvements include higher drive
capability, 5V I/O tolerance, lower supply current, individual I/O
configuration, and smaller packaging. I/O expanders provide a
simple solution when additional I/O is needed for ACPI power
switches, sensors, pushbuttons, LEDs, fans, etc.
The PCA9555 consist of two 8-bit Configuration (Input or Output
selection); Input, Output and Polarity inversion (Active high or Active
low operation) registers. The system master can enable the I/Os as
either inputs or outputs by writing to the I/O configuration bits. The
data for each Input or Output is kept in the corresponding Input or
Output register. The polarity of the read register can be inverted with
the Polarity Inversion Register. All registers can be read by the
system master. Although pin to pin and I2C address compatible with
the PCF8575, software changes are required due to the
enhancements and are discussed in Application Note AN469.
The PCA9555 open-drain interrupt output is activated when any
input state differs from its corresponding input port register state and
is used to indicate to the system master that an input state has
changed. The power-on reset sets the registers to their default
values and initializes the device state machine.
Three hardware pins (A0, A1, A2) vary the fixed I2C address and
allow up to eight devices to share the same I2C/SMBus. The fixed
I2C address of the PCA9555 is the same as the PCA9554 allowing
up to eight of these devices in any combination to share the same
I2C/SMBus.