REMOTE 16-BIT I
2
C AND SMBus I/O EXPANDER
WITH INTERRUPT OUTPUT AND CONFIGURATION REGISTERS
SCPS131D – AUGUST 2005 – REVISED OCTOBER 2006
www.ti.com
DESCRIPTION/ORDERING INFORMATION (CONTINUED)
This 16-bit I/O expander for the two-line bidirectional bus (I
2
C) is designed for 2.3-V to 5.5-V V
CC
operation. It
provides general-purpose remote I/O expansion for most microcontroller families via the I
2
C 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 I
2
C/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 I
2
C bus. Thus, the PCA9555 can remain a simple slave device.
The device outputs (latched) have high-current drive capability for directly driving LEDs.
Although pin-to-pin and I
2
C-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 undriven.
Three hardware pins (A0, A1, and A2) are used to program and vary the fixed I
2
C address and allow up to eight
devices to share the same I
2
C bus or SMBus. The fixed I
2
C 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 I
2
C bus or SMBus.
2
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