NXP Semiconductors

PCA9574

8-bit I2C-bus and SMBus, level translating, low voltage GPIO with reset and interrupt

corresponding thermal pad on the board and for proper heat conduction through the board, thermal vias need to be incorporated in the PCB in the thermal

pad region.

7 Functional description

7.1 Device address

Following a START condition the bus controller must send the address of the target it is accessing and the

operation it wants to perform (read or write). The address of the PCA9574 is shown in Figure 5. Target address

pin A0 chooses 1 of 2 target addresses: 40h or 42h.

002aad055

0

1

0

0

0

0

A0 R/W

fixed

target address

hardware selectable

Figure 5. PCA9574 device address

The last bit of the first byte defines the operation to be performed. When set to logic 1 a read is selected, while

logic 0 selects a write operation.

7.2 Command register

Following the successful acknowledgement of the target address + R/W bit, the bus controller will send a byte to

the PCA9574, which will be stored in the Command register.

002aad056

AI

X

X

X

X

D2 D1 D0

register address

Auto-Increment flag

Reset state = 00h

Remark: The Command register does not apply to Software Reset I2C-bus address.

Figure 6. Command register

The lowest three bits are used as a pointer to determine which register will be accessed. Only a command

register code with the three least significant bits equal to the eight allowable values as defined in Table 4 will be

acknowledged. Reserved or undefined command codes will not be acknowledged. At power-up, this register

defaults to 00h, with the AI bit set to ‘0’, and the lowest 3 bits set to ‘0’.

If the Auto-Increment flag is set (AI = 1), the three least significant bits of the Command register are

automatically incremented after a read or write. This allows the user to program and/or read the eight command

registers (listed in Table 4) sequentially. It will then roll over to register 00h after the last register is accessed and

the selected registers will be overwritten or re-read.

If the Auto-Increment flag is cleared (AI = 0), the three least significant bits are not incremented after data is

read or written, only one register will be repeatedly read or written.

7.3 Register definitions

Register

number

D2

D1

D0

Name

Type

Function

00h

0

0

0

IN

read only

Input port register

Table 4. Register summary

PCA9574

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Product data sheet

Rev. 6.0 — 31 May 2023

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