The PCA9555 is a 24-pin CMOS device that provides 16 bits of General Purpose parallel
Input/Output (GPIO) expansion for I2C-bus/SMBus applications and was 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, 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 PCA9555 consists 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-bus
address compatible with the PCF8575, software changes are required due to the
enhancements, and are discussed in Application Note AN469.

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