PCA9538DBRG4 datasheet(7/29 Pages) TI

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Power-On Reset

RESET Input

PCA9538

REMOTE 8-BIT I2C AND SMBus LOW-POWER I/O EXPANDER

WITH INTERRUPT OUTPUT, RESET, AND CONFIGURATION REGISTERS

SCPS126D – SEPTEMBER 2006 – REVISED MARCH 2007

The Input Port register (register 0) reflects the incoming logic levels of the pins, regardless of whether the pin is

defined as an input or an output by the Configuration register. It only acts on read operation. Writes to these

registers have no effect. The default value, X, is determined by the externally applied logic level.

Before a read operation, a write transmission is sent with the command byte to indicate to the I2C device that the

Input Port register will be accessed next.

BIT

DEFAULT

The Output Port register (register 1) shows the outgoing logic levels of the pins defined as outputs by the

Configuration register. Bit values in this register have no effect on pins defined as inputs. In turn, reads from this

BIT

DEFAULT

The Polarity Inversion register (register 2) allows polarity inversion of pins defined as inputs by the Configuration

register. If a bit in this register is set (written with 1), the corresponding port pin polarity is inverted. If a bit in this

BIT

DEFAULT

The Configuration register (register 3) configures the directions of the I/O pins. If a bit in this register is set to 1,

the corresponding port pin is enabled as an input with a high-impedance output driver. If a bit in this register is

cleared to 0, the corresponding port pin is enabled as an output.

BIT

DEFAULT

When power (from 0 V) is applied to VCC, an internal power-on reset holds the PCA9538 in a reset condition

until VCC has reached VPOR. At that point, the reset condition is released and the PCA9538 registers and

SMBus/I2C state machine will initialize to their default states. After that, VCC must be lowered to below 0.2 V and

then back up to the operating voltage for a power-reset cycle.

The RESET input can be asserted to reset the system while keeping the VCC at its operating level. A reset can

be accomplished by holding the RESET pin low for a minimum of tW. The PCA9538 registers and I

2C/SMBus

state machine are changed to their default states once RESET is low (0). Once RESET is high (1), the I/O levels

at the P port can be changed externally or through the master. This input requires a pullup resistor to VCC if no

active connection is used.

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