SDA

SCL

Start Condition

S

Stop Condition

P

PCF8575C

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SCPS123F – MARCH 2005 – REVISED JANUARY 2015

8.3 Feature Description

connected to a positive supply via a pullup resistor when connected to the output stages of a device. Data

transfer may be initiated only when the bus is not busy.

the SDA input/output while the SCL input is high (see Figure 10). After the start condition, the device address

byte is sent, MSB first, including the data direction bit (R/W). This device does not respond to the general call

address. After receiving the valid address byte, this device responds with an ACK, a low on the SDA input/output

during the high of the ACK-related clock pulse. The address inputs (A2–A0) of the slave device must not be

changed between the start and the stop conditions.

The data byte follows the address ACK. If the R/W bit is high, the data from this device are the values read from

the P port. If the R/W bit is low, the data are from the master, to be output to the P port. The data byte is followed

by an ACK sent from this device. If other data bytes are sent from the master, following the ACK, they are

ignored by this device. Data are output only if complete bytes are received and acknowledged. The output data is

stable during the high pulse of the clock period, as changes in the data line at this time are interpreted as control

commands (start or stop) (see Figure 11).

A stop condition, a low-to-high transition on the SDA input/output while the SCL input is high, is sent by the

master (see Figure 10).

The number of data bytes transferred between the start and the stop conditions from transmitter to receiver is not

limited. Each byte of eight bits is followed by one ACK bit. The transmitter must release the SDA line before the

receiver can send an ACK bit.

A slave receiver that is addressed must generate an ACK after the reception of each byte. Also, a master must

generate an ACK after the reception of each byte that has been clocked out of the slave transmitter. The device

that acknowledges has to pull down the SDA line during the ACK clock pulse so that the SDA line is stable low

during the high pulse of the ACK-related clock period (see Figure 12). Setup and hold times must be taken into

account.

A master receiver must signal an end of data to the transmitter by not generating an acknowledge (NACK) after

the last byte that has been clocked out of the slave. This is done by the master receiver by holding the SDA line

high. In this event, the transmitter must release the data line to enable the master to generate a stop condition.

Figure 10. Definition of Start and Stop Conditions

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