SCPS131E – AUGUST 2005 – REVISED MAY 2008
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Device Address
shows the address byte of the PCA9555.
R/W
Slave Address
0
1
0
0 A2 A1 A0
Fixed
Programmable
Figure 4. PCA9555 Address
Address Reference
INPUTS
A2
L
L
L
L
H
H
H
H
A1
L
L
H
H
L
L
H
H
A0
L
H
L
H
L
H
L
H
I
2
C BUS SLAVE ADDRESS
32 (decimal), 20 (hexadecimal)
33 (decimal), 21 (hexadecimal)
34 (decimal), 22 (hexadecimal)
35 (decimal), 23 (hexadecimal)
36 (decimal), 24 (hexadecimal)
37 (decimal), 25 (hexadecimal)
38 (decimal), 26 (hexadecimal)
39 (decimal), 27 (hexadecimal)
The last bit of the slave address defines the operation (read or write) to be performed. A high (1) selects a read
operation, while a low (0) selects a write operation.
Control Register and Command Byte
Following the successful acknowledgment of the address byte, the bus master sends a command byte that is
stored in the control register in the PCA9555. Three bits of this data byte state the operation (read or write) and
the internal register (input, output, polarity inversion, or configuration) that will be affected. This register can be
written or read through the I
2
C bus. The command byte is sent only during a write transmission.
Once a command byte has been sent, the register that was addressed continues to be accessed by reads until a
new command byte has been sent.
0
0
0
0
0
B2
B1
B0
Figure 5. Control Register Bits
Command Byte
CONTROL REGISTER BITS
B2
0
0
0
0
1
1
1
1
B1
0
0
1
1
0
0
1
1
B0
0
1
0
1
0
1
0
1
COMMAND
BYTE (HEX)
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
REGISTER
Input Port 0
Input Port 1
Output Port 0
Output Port 1
Polarity Inversion Port 0
Polarity Inversion Port 1
Configuration Port 0
Configuration Port 1
PROTOCOL
Read byte
Read byte
Read/write byte
Read/write byte
Read/write byte
Read/write byte
Read/write byte
Read/write byte
POWER-UP
DEFAULT
xxxx xxxx
xxxx xxxx
1111 1111
1111 1111
0000 0000
0000 0000
1111 1111
1111 1111
8
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