The PCA9554 and PCA9554A are 16-pin CMOS devices that provide 8 bits of General
Purpose parallel Input/Output (GPIO) expansion for I2C-bus/SMBus applications and
were developed to enhance the NXP Semiconductors family of I2C-bus I/O expanders.
The improvements include higher drive capability, 5 V I/O tolerance, lower supply current,
individual I/O configuration, 400 kHz clock frequency, and smaller packaging. I/O
expanders provide a simple solution when additional I/O is needed for ACPI power
switches, sensors, push buttons, LEDs, fans, etc.
The PCA9554/PCA9554A consist of an 8-bit Configuration register (Input or Output
selection); 8-bit Input Port register, 8-bit Output Port register and an 8-bit Polarity
Inversion register (active HIGH or active LOW operation). 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 Port or Output Port 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-bus address
compatible with the PCF8574 series, software changes are required due to the
enhancements and are discussed in Application Note AN469.
The PCA9554/PCA9554A 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-bus address and allow up to eight
devices to share the same I2C-bus/SMBus. The PCA9554A is identical to the PCA9554
except that the fixed I2C-bus address is different allowing up to sixteen of these devices
(eight of each) on the same I2C-bus/SMBus.