PCA9554

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7

Functional Description

The PCA9554’s general purpose input/output (GPIO)

peripherals provide up to eight I/O ports, controlled through

sends data to the bus to be a transmitter and any device

receiving data to be a receiver. The transfer is controlled by

the Master device which generates the serial clock and all

START and STOP conditions for bus access. The PCA9554

operates as a Slave device. Both the Master device and Slave

device can operate as either transmitter or receiver, but the

Master device controls which mode is activated.

1. Data transfer may be initiated only when the bus is

not busy.

2. During a data transfer, the data line must remain

stable whenever the clock line is high. Any

changes in the data line while the clock line is high

will be interpreted as a START or STOP condition

(Figure 6).

START and STOP Conditions

The START Condition precedes all commands to the

device, and is defined as a HIGH to LOW transition of SDA

when SCL is HIGH. The PCA9554 monitors the SDA and

SCL lines and will not respond until this condition is met.

A LOW to HIGH transition of SDA when SCL is HIGH

determines the STOP condition. All operations must end

with a STOP condition.

Device Addressing

After the bus Master sends a START condition, a slave

address byte is required to enable the PCA9554 for a read or

write operation. The four most significant bits of the slave

address are fixed as binary 0100 for the PCA9554 (Figure 7).

The PCA9554 uses the next three bits as address bits.

The address bits A2, A1 and A0 are used to select which

device is accessed from maximum eight devices on the same

bus. These bits must compare to their hardwired input pins.

The 8th bit following the 7−bit slave address is the R/W bit

that specifies whether a read or write operation is to be

performed. When this bit is set to “1”, a read operation is

initiated, and when set to “0”, a write operation is selected.

Following the START condition and the slave address

byte, the PCA9554 monitors the bus and responds with an

acknowledge (on the SDA line) when its address matches

the transmitted slave address. The PCA9554 then performs

a read or a write operation depending on the state of the R/W

bit.

START

CONDITION

STOP

CONDITION

SDA

SCL

Figure 6. START/STOP Condition

0

1

0

0

A2

A1

A0

SLAVE ADDRESS

FIXED

PROGRAMMABLE

HARDWARE

SELECTABLE

Figure 7. PCA9554 Slave Address

R/W