Command Byte; Pca9554 Register 0; Pca9554 Register 1 - Output Port Register - Advantech ITA-5231 Series User Manual

6.2.1

Command Byte

Table 6.3: Command Byte
Command
0
1
2
3
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. PCA9554 has a total of four registers to control GPIO.
6.2.2

PCA9554 Register 0

This register is read-only and reflects the incoming logic levels of the pins, regardless
of whether the pin is defined as an input or an output by Register 3. Writing to the
register has no effect. The default "X" is determined by the externally applied logic
level, which is normal "1" when no external signal is applied because of the internal
pull-up resistors.
Table 6.4: Register 0 – Input Port Register Bit Description
Bit Symbol
7 I7
6 I6
5 I5
4 I4
3 I3
2 I2
1 I1
0 I0
If one GPIO pin is set to input, the input value can be read from the bit that corre-
sponds to Register 0.
6.2.3 PCA9554 Register 1 – Output Port Register
This register reflects the outgoing logic levels of the pins defined as outputs by Regis-
ters 3. Bit values in this register have no effect on pins defined as inputs. Reads from
this register return the value that is in the flip-flop controlling the output selection, not
the actual pin value.
Table 6.5: Register 1 – Output Port Register Bit Description
Bit Symbol
7 O7
6 O6
5 O5
4 O4
3 O3
2 O2
1 O1
0 O0
Protocol
Read byte
Read/write byte
Read/write byte
Read/write byte
Access Value Description
Read only X Determined by externally applied logic level
Read only X
Read only X
Read only X
Read only X
Read only X
Read only X
Read only X
Access Value
R 1*
R 1*
R 1*
R 1*
R 1*
R 1*
R 1*
R 1*
65
Function
Input port register
Output port register
Polarity inversion register
Configuration register
Description
Determined by externally applied logic level
ITA-5231 User Manual