December 1994 8
Philips Semiconductors Product specification
256 x 8-bit CMOS EEPROMS
with I
2
C-bus interface
PCX8582X-2 Family
I
2
C-BUS PROTOCOL
The I
2
C-bus is for 2-way, 2-line communication between
different ICs or modules. The serial bus consists of two
bidirectional lines: one for data signals (SDA), and one for
clock signals (SCL).
Both the SDA and SCL lines must be connected to a
positive supply voltage via a pull-up resistor.
The following protocol has been defined:
• Data transfer may be initiated only when the bus is not
busy.
• During data transfer, the data line must remain stable
whenever the clock line is HIGH. Changes in the data
line while the clock line is HIGH will be interpreted as
control signals.
The following bus conditions have been defined:
• Bus not busy: both data and clock lines remain HIGH.
• Start data transfer: a change in the state of the data
line, from HIGH-to-LOW, while the clock is HIGH,
defines the start condition.
• Stop data transfer: a change in the state of the data
line, from LOW-to-HIGH, while the clock is HIGH,
defines the stop condition.
• Data valid: the state of the data line represents valid
data when, after a start condition, the data line is stable
for the duration of the HIGH period of the clock signal.
There is one clock pulse per bit of data.
Each data transfer is initiated with a start condition and
terminated with a stop condition; the number of the data
bytes, transferred between the start and stop conditions is
limited to seven bytes in the ERASE/WRITE mode and
eight bytes in the PAGE ERASE/WRITE mode. Data
transfer is unlimited in the READ mode. The information is
transmitted in bytes and each receiver acknowledges with
a ninth bit.
Within the I
2
C-bus specifications a low-speed mode (2 kHz
clock rate) and a high speed mode (100 kHz clock rate)
are defined.
The PCX8582X-2 operates in both modes.
By definition a device that sends a signal is called a
‘transmitter’, and the device which receives the signal is
called a ‘receive’. The device which controls the signal is
called the ‘master’. The devices that are controlled by the
master are called ‘slaves’.
Each byte is followed by one acknowledge bit. This
acknowledge bit is a HIGH level, put on the bus by the
transmitter. The master generates an extra acknowledge
related clock pulse. The slave receiver which is addressed
is obliged to generate an acknowledge after the reception
of each byte.
The master receiver must generate an acknowledge 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 acknowledge clock pulse in such a way that
the SDA line is stable LOW during the HIGH period of the
acknowledge related clock pulse.
Set-up and hold times must be taken into account. A
master receiver must signal an end of data to the slave
transmitter by not generating an acknowledge on the last
byte that has been clocked out of the slave. In this event
the transmitter must leave the data line HIGH to enable the
master generation of the stop condition.
DEVICE ADDRESSING
Following a start condition the bus master must output the
address of the slave it is accessing. The most significant
four bits of the slave address are the device type identifier
(see Fig.3). For the PCX8582X-2 this is fixed as 1010.
The next three significant bits address a particular device.
A system could have up to eight PCX8582X-2 devices on
the bus. The eight addresses are defined by the state of
the A0, A1 and A2 inputs.
The last bit of the slave address defines the operation to
be performed. When set to logic 1 a read operation is
selected.
Address bits must be connected to either V
DD
or V
SS
.
Fig.3 Slave address.
handbook, halfpage
MBC793
1 0 1 0 A2 A1 A0 R/W
