AXIOMTEK NANO820, User Manual

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NANO840/842 Nano-ITX Board
Digital I/O 61
Appendix B
Digital I/O
B.1 Digital I/O Software Programming
 I
2
C to GPIO PCA9554PW GPIO.
 I
2
C address: 01001000.
Command byte
Command Protocol Function
0 Read byte Input port register
1 Read/write byte Output port register
2 Read/write byte Polarity inversion register
3 Read/write byte Configuration register
The command byte is the first byte to follow the address byte during a write transmission. It is
used as a pointer to determine which of the following registers will be written or read.
Register 0: Input port register.
This register is a read-only port. It reflects the incoming logic levels of the pins, regardless of
whether the pin is defined as an input or an output by Register 3. Writes to this register have
no effect.
The default ‘X’ is determined by the externally applied logic level, normally ‘1’ when no external
signal externally applied because of the internal pull-up resistors.
Register 0
–
Input port register bit description
Bit Symbol Access Value Description
7 I7 Read only X
Determined by externally applied
logic level.
6 I6 Read only X
5 I5 Read only X
4 I4 Read only X
3 I3 Read only X
2 I2 Read only X
1 I1 Read only X
0 I0 Read only X