®
IS24C256
ISSI
Standby Mode
DEVICE OPERATION
The IS24C256 features a serial communication and
supports a bi-directional 2-wire bus transmission protocol.
Power consumption in reduced in standby mode. The
IS24C256 will enter standby mode: a) At Power-up, and
remaininituntilSCLorSDAtoggles;b)FollowingtheStop
signal if no write operation is initiated; or c) Following any
internalwriteoperation
2-WIRE BUS
The two-wire bus is defined as a Serial Data line (SDA),
and a Serial Clock line (SCL). The protocol defines any
device that sends data onto the SDA bus as a transmitter,
and the receiving devices as a receiver. The bus is
controlled by Master device which generates the SCL,
controls the bus access and generates the Stop and Start
conditions. The IS24C256 is the Slave device on the bus.
DEVICE ADDRESSING
The Master begins a transmission by sending a Start
condition. The Master then sends the address of the
particular Slave devices it is requesting. The Slave
(Fig. 5) address is 8 bits.
The Bus Protocol:
The four most significant bits of the address are fixed as
1010 for the IS24C256.
– Data transfer may be initiated only when the bus is not
busy
This device has three address bits (A1, A2, and A0),
which allows up to eight IS24C256 devices to share the
2-wire bus. Upon receiving the Slave address, the
device compares the three address bits with the
hardwired A2, A1, and A0 input pins to determine if it is
the appropriate Slave. If the A2, A1, and A0 pins are
not biased to High nor Low, then internal circuitry
– During a data transfer, the data line must remain stable
whenever the clock line is high. Any changes in the
data line while the clock line is high will be interpreted
as a Start or Stop condition.
The state of the data line represents valid data after a Start
condition. The data line must be stable for the duration of
the High period of the clock signal. The data on the SDA
line may be changed during the Low 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.
defaults the value to Low.
The last bit of the Slave address specifies whether a Read
or Write operation is to be performed. When this bit is set
to 1, a Read operation is selected, and when set to 0, a
Write operation is selected.
Start Condition
After the Master transmits the Start condition and
Slave address byte (Fig. 5), the appropriate 2-wire
Slave (eg. IS24C256) will respond with ACK on the
SDA line. The Slave will pull down the SDA on the
ninth clock cycle, signaling that it received the eight
bits of data. The selected IS24C256 then prepares for a
The Start condition precedes all commands to the device
and is defined as a High to Low transition of SDA when
SCL is High. The IS24C256 monitors the SDA and SCL
lines and will not respond until the Start condition is met.
Stop Condition
Read or Write operation by monitoring the bus.
The Stop condition is defined as a Low to High transition
of SDA when SCL is High. All operations must end with
a Stop condition.
WRITE OPERATION
Byte Write
Acknowledge (ACK)
In the Byte Write mode, the Master device sends the Start
condition and the Slave address information (with the R/W
settoZero)totheSlavedevice. AftertheSlavegenerates
an ACK, the Master sends two byte addresses that are to
be written into the address pointer of the IS24C256. After
receiving another ACK from the Slave, the Master device
transmits the data byte to be written into the address
memorylocation.TheIS24C256acknowledgesoncemore
andtheMastergeneratestheStopcondition,atwhichtime
the device begins its internal programming cycle. While
thisinternalcycleisinprogress,thedevicewillnotrespond
to any request from the Master device.
After a successful data transfer, each receiving device is
required to generate an ACK. The Acknowledging device
pulls down the SDA line.
Reset
The IS24C256 contains a reset function in case the 2-
wire bus transmission is accidentally interrupted (eg. a
power loss), or needs to be terminated mid-stream.
The reset is caused when the Master device creates a
Start condition. To do this, it may be necessary for the
Master device to monitor the SDA line while cycling the
SCL up to nine times. (For each clock signal transition
to High, the Master checks for a High level on SDA.)
4
Integrated Silicon Solution, Inc. — www.issi.com — 1-800-379-4774
ADVANCEDINFORMATION Rev. 00A
03/11/03
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