PCF8563
NXP Semiconductors
Real-time clock/calendar
9. Characteristics of the I2C-bus
The I2C-bus is for bidirectional, two-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.
9.1 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 a control signal (see Figure 14).
SDA
SCL
data line
stable;
data valid
change
of data
allowed
mbc621
Fig 14. Bit transfer
9.2 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 15).
SDA
SCL
SDA
SCL
S
P
START condition
STOP condition
mbc622
Fig 15. Definition of START and STOP conditions
9.3 System configuration
A device generating a message is a transmitter; a device receiving a message is a
receiver. The device that controls the message is the master; and the devices which are
controlled by the master are the slaves (see Figure 16).
PCF8563
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© NXP B.V. 2011. All rights reserved.
Product data sheet
Rev. 9 — 16 June 2011
21 of 45
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