Advantech ITA-1611 Series User Manual page 60

PCA9554 register 1:
This register reflects outgoing logic level of pins defined as outputs by Register 3.
The Bit value in this register will not respond to input pin. When reading this register,
value of flip-flop which controls output will be returned, while the actual pin's physical
signal value will not be returned.
If a certain GPIO Pin is set to Output. you can read corresponding Output value by
reading corresponding bit of register 1.
PCA9554 register 2:
This register allows the user to invert polarity of Input Port register data. If value of a
certain bit is set (marked as "1"), data polarity of corresponding Input Port will be
inverted. If the values is cleared (marked as "0"), the Input Port data remains the pre-
vious polarity.
If a certain GPIO Pin is set to Input. you can set corresponding bit of register 2 to con-
trol Input pin.
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