This 16-bit I/O expander for the two-line bidirectional bus
(I2C) is designed for 2.3-V to 5.5-V VCC
operation. It provides general-purpose remote I/O expansion for most microcontroller families via
the I2C interface [serial clock (SCL), serial data (SDA)].
The PCA9555 consists of two 8-bit Configuration (input or output selection), Input Port,
Output Port, and Polarity Inversion (active high or active low operation) registers. At power on,
the I/Os are configured as inputs. 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 Input Port register can be inverted
with the Polarity Inversion register. All registers can be read by the system master.
The system master can reset the PCA9555 in the event of a timeout or other improper
operation by utilizing the power-on reset feature, which puts the registers in their default state
and initializes the I2C/SMBus state machine.
The PCA9555 open-drain interrupt (INT) 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.
INT can be connected to the interrupt input of a microcontroller.
By sending an interrupt signal on this line, the remote I/O can inform the microcontroller if there
is incoming data on its ports without having to communicate via the I2C
bus. Thus, the PCA9555 can remain a simple slave device.
The device outputs (latched) have high-current drive capability for directly driving
Although pin-to-pin and I2C-address is compatible with the
PCF8575, software changes are required due to the enhancements.
The PCA9555 is identical to the PCA9535, except for the inclusion of the internal I/O
pullup resistor, which pulls the I/O to a default high when configured as an input and
Three hardware pins (A0, A1, and A2) are used to program and vary the fixed
I2C address and allow up to eight devices to share the same
I2C bus or SMBus. The fixed I2C address of
the PCA9555 is the same as the PCF8575, PCF8575C, and PCF8574, allowing up to eight of these
devices in any combination to share the same I2C bus or SMBus.
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