CAT24C256
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6
Power-On Reset (POR)
The CAT24C256 Die Rev. C incorporates Power−On
Reset (POR) circuitry which protects the internal logic
against powering up in the wrong state.
The device will power up into Standby mode after V
CC
exceeds the POR trigger level and will power down into
Reset mode when V
CC
drops below the POR trigger level.
This bi−directional POR behavior protects the device
against brown−out failure, following a temporary loss of
power.
Pin Description
SCL: The Serial Clock input pin accepts the Serial Clock
signal generated by the Master.
SDA: The Serial Data I/O pin receives input data and
transmits data stored in EEPROM. In transmit mode, this pin
is open drain. Data is acquired on the positive edge, and is
delivered on the negative edge of SCL.
A
0
, A
1
and A
2
: The Address pins accept the device address.
These pins have on−chip pull−down resistors.
WP: The Write Protect input pin inhibits all write
operations, when pulled HIGH. This pin has an on−chip
pull−down resistor.
Functional Description
The CAT24C256 supports the Inter−Integrated Circuit
(I
2
C) Bus data transmission protocol, which defines a device
that sends data to the bus as a transmitter and a device
receiving data as a receiver. Data flow is controlled by a
Master device, which generates the serial clock and all
START and STOP conditions. The CAT24C256 acts as a
Slave device. Master and Slave alternate as either
transmitter or receiver. Up to 8 devices may be connected to
the bus as determined by the device address inputs A
0
, A
1
,
and A
2
.
I
2
C Bus Protocol
The I
2
C bus consists of two ‘wires’, SCL and SDA. The
two wires are connected to the V
CC
supply via pull−up
resistors. Master and Slave devices connect to the 2−wire
bus via their respective SCL and SDA pins. The transmitting
device pulls down the SDA line to ‘transmit’ a ‘0’ and
releases it to ‘transmit’ a ‘1’.
Data transfer may be initiated only when the bus is not
busy (see A.C. Characteristics).
During data transfer, the SDA line must remain stable
while the SCL line is HIGH. An SDA transition while SCL
is HIGH will be interpreted as a START or STOP condition
(Figure 2).
START
The START condition precedes all commands. It consists
of a HIGH to LOW transition on SDA while SCL is HIGH.
The START acts as a ‘wake−up’ call to all receivers. Absent
a START, a Slave will not respond to commands.
STOP
The STOP condition completes all commands. It consists
of a LOW to HIGH transition on SDA while SCL is HIGH.
The STOP starts the internal Write cycle (when following a
Write command) or sends the Slave into standby mode
(when following a Read command).
Device Addressing
The Master initiates data transfer by creating a START
condition on the bus. The Master then broadcasts an 8−bit
serial Slave address. The first 4 bits of the Slave address are
set to 1010, for normal Read/Write operations (Figure 3).
The next 3 bits, A
2
, A
1
and A
0
, select one of 8 possible Slave
devices. The last bit, R/W, specifies whether a Read (1) or
Write (0) operation is to be performed.
Acknowledge
After processing the Slave address, the Slave responds
with an acknowledge (ACK) by pulling down the SDA line
during the 9th clock cycle (Figure 4). The Slave will also
acknowledge the byte address and every data byte presented
in Write mode. In Read mode the Slave shifts out a data byte,
and then releases the SDA line during the 9th clock cycle. If
the Master acknowledges the data, then the Slave continues
transmitting. The Master terminates the session by not
acknowledging the last data byte (NoACK) and by sending
a STOP to the Slave. Bus timing is illustrated in Figure 5.
