PCA9549_2 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 02 — 13 July 2009 8 of 25
NXP Semiconductors
PCA9549
Octal bus switch with individually I
2
C-bus controlled enables
7. Characteristics of the I
2
C-bus
The I
2
C-bus is for 2-way, 2-line communication between different ICs or modules. The two
lines are a serial data line (SDA) and a serial clock line (SCL). Both lines must be
connected to a positive supply via a pull-up resistor. Data transfer may be initiated only
when the bus is not busy.
7.1 Bit transfer
One data bit is transferred during each clock pulse. The data on the SDA line must remain
stable during the HIGH period of the clock pulse as changes in the data line at this time
will be interpreted as control signals (see Figure 8).
7.1.1 START and STOP conditions
Both data and clock lines remain HIGH when the bus is not busy. A HIGH-to-LOW
transition of the data line while the clock is HIGH is defined as the START condition (S).
A LOW-to-HIGH transition of the data line while the clock is HIGH is defined as the STOP
condition (P) (see Figure 9).
7.2 System configuration
A device generating a message is a ‘transmitter’; a device receiving is the ‘receiver’. The
device that controls the message is the ‘master’ and the devices which are controlled by
the master are the ‘slaves’ (see Figure 10).
Fig 8. Bit transfer
mba607
data line
stable;
data valid
change
of data
allowed
SDA
SCL
Fig 9. Definition of START and STOP conditions
mba608
SDA
SCL
P
STOP condition
S
START condition
