Philips Semiconductors

Product data sheet

PCA9539

with interrupt and reset

2004 Sep 30

7

BUS TRANSACTIONS

Writing to the port registers

Data is transmitted to the PCA9539 by sending the device address

and setting the least significant bit to a logic 0 (see Figure 5 for

device address). The command byte is sent after the address and

determines which register will receive the data following the

command byte.

The eight registers within the PCA9539 are configured to operate

as four register pairs. The four pairs are Input Ports, Output Ports,

Polarity Inversion Ports, and Configuration Ports. After sending data

to one register, the next data byte will be sent to the other register in

the pair (see Figures 6 and 7). For example, if the first byte is sent to

Output Port (register 3), then the next byte will be stored in Output

Port 0 (register 2). There is no limitation on the number of data bytes

sent in one write transmission. In this way, each 8-bit register may

be updated independently of the other registers.

Reading the port registers

In order to read data from the PCA9539, the bus master must first

send the PCA9539 address with the least significant bit set to a

logic 0 (see Figure 5 for device address). The command byte is sent

after the address and determines which register will be accessed.

After a restart, the device address is sent again but this time, the

least significant bit is set to a logic 1. Data from the register defined

by the command byte will then be sent by the PCA9539 (see

Figures 8 , 9, and 10). Data is clocked into the register on the falling

edge of the acknowledge clock pulse. After the first byte is read,

additional bytes may be read but the data will now reflect the

information in the other register in the pair. For example, if you read

Input Port 1, then the next byte read would be Input Port 0. There is

no limitation on the number of data bytes received in one read

transmission but the final byte received, the bus master must not

acknowledge the data.

Interrupt Output

The open-drain interrupt output is activated when one of the port

pins change state and the pin is configured as an input. The

interrupt is deactivated when the input returns to its previous state or

the input port register is read (see Figure 9). A pin configured as an

output cannot cause an interrupt. Since each 8-bit port is read

independently, the interrupt caused by Port 0 will not be cleared by a

read of Port 1 or the other way around.

Note that changing an I/O from an output to an input may cause a

false interrupt to occur if the state of the pin does not match the

contents of the Input Port register.