arising out of the application or use of any information, products or services described herein. Customers are advised to obtain the latest version of this device specification before relying on any
published information and before placing orders for products.
Integrated Silicon Solution, Inc. — www.issi.com —
1-800-379-4774
ADVANCED INFORMATION Rev. 00B
03/11/03
1
IS24C256
ISSI
®
FUNCTIONAL BLOCK DIAGRAM
Vcc
HIGH VOLTAGE
GENERATOR,
TIMING & CONTROL
SDA
WP
SLAVE ADDRESS
REGISTER &
COMPARATOR
A0
A1
A2
WORD ADDRESS
COUNTER
X
DECODER
SCL
CONTROL
LOGIC
EEPROM
ARRAY
Y
DECODER
GND
nMOS
ACK
Clock
DI/O
>
DATA
REGISTER
2
Integrated Silicon Solution, Inc. — www.issi.com —
1-800-379-4774
ADVANCED INFORMATION Rev. 00A
03/11/03
IS24C256
ISSI
14-pin TSSOP
®
PIN CONFIGURATION
8-Pin DIP and SOIC
A0
A0
A1
A2
GND
1
2
3
4
8
7
6
5
VCC
WP
SCL
SDA
A1
A2
NC
NC
NC
GND
1
2
3
4
5
6
7
14
13
12
11
10
9
8
VCC
WP
NC
NC
NC
SCL
SDA
PIN DESCRIPTIONS
A0-A2
SDA
SCL
WP
Vcc
NC
GND
Address Inputs
Serial Address/Data I/O
Serial Clock Input
Write Protect Input
Power Supply
No Connect
Ground
drain output and can be wire Or'ed with other open drain
or open collector outputs. The SDA bus
requires
a pullup
resistor to Vcc.
A0, A1, A2
The A0, A1, and A2 are the device address inputs that are
hardwired or left not connected for hardware compatibility
with the IS24C32/64/128. When pins are hardwired, as
many as eight 256K devices may be addressed on a single
bus system. When the pins are not hardwired, the default
A0, A1, and A2 are zero.
SCL
This input clock pin is used to synchronize the data
transfer to and from the device.
WP
WP is the Write Protect pin. If the WP pin is tied to Vcc
the entire array becomes Write Protected (Read only).
When WP is tied to GND or left floating, normal read/write
operations are allowed to the device.
SDA
The SDA is a Bi-directional pin used to transfer addresses
and data into and out of the device. The SDA pin is an open
Integrated Silicon Solution, Inc. — www.issi.com —
1-800-379-4774
ADVANCED INFORMATION Rev. 00A
03/11/03
3
IS24C256
DEVICE OPERATION
The IS24C256 features a serial communication and
supports a bi-directional 2-wire bus transmission protocol.
ISSI
Standby Mode
®
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.
Power consumption in reduced in standby mode. The
IS24C256 will enter standby mode: a) At Power-up, and
remain in it until SCL or SDA toggles; b) Following the Stop
signal if no write operation is initiated; or c) Following any
internal write operation
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 four most significant bits of the address are fixed as
1010 for the IS24C256.
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
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.
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
Read or Write operation by monitoring the bus.
The Bus Protocol:
– Data transfer may be initiated only when the bus is not
busy
– 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.
Start Condition
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
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
In the Byte Write mode, the Master device sends the Start
condition and the Slave address information (with the R/W
set to Zero) to the Slave device. After the Slave generates
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
memory location. The IS24C256 acknowledges once more
and the Master generates the Stop condition, at which time
the device begins its internal programming cycle. While
this internal cycle is in progress, the device will not respond
to any request from the Master device.
Acknowledge (ACK)
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
ADVANCED INFORMATION Rev. 00A
03/11/03
IS24C256
Page Write
The IS24C256 is capable of 64-byte Page-Write operation. A
Page-Write is initiated in the same manner as a Byte Write,
but instead of terminating the internal Write cycle after the
first data word is transferred, the Master device can transmit
up to 63 more bytes. After the receipt of each data word, the
IS24C256 responds immediately with an ACK on SDA line,
and the six lower order data word address bits are internally
incremented by one, while the higher order bits of the data
word address remain constant. If the Master device should
transmit more than 64 words, prior to issuing the Stop
condition, the address counter will “roll over,” and the previously
written data will be overwritten. Once all 64 bytes are
received and the Stop condition has been sent by the Master,
the internal programming cycle begins. At this point, all
received data is written to the IS24C256 in a single Write
cycle. All inputs are disabled until completion of the internal
Write cycle.
ISSI
Random Address Read
®
Selective Read operations allow the Master device to
select at random any memory location for a Read
operation. The Master device first performs a 'dummy'
Write operation by sending the Start condition, Slave
address and word address of the location it wishes to
read. After the IS24C256 acknowledges the word address,
the Master device resends the Start condition and the
Slave address, this time with the R/W bit set to one. The
IS24C256 then responds with its ACK and sends the data
requested. The Master device does not send an ACK but
will generate a Stop condition. (Refer to Figure 9. Random
Address Read Diagram.)
Sequential Read
Sequential Reads can be initiated as either a Current
Address Read or Random Address Read. After the
IS24C256 sends initial byte sequence, the Master device
now responds with an ACK indicating it requires additional
data from the IS24C256. The IS24C256 continues to
output data for each ACK received. The Master device
terminates the sequential Read operation by pulling SDA
High (no ACK) indicating the last data word to be read,
followed by a Stop condition.
The data output is sequential, with the data from address
n followed by the data from address n+1, ... etc. The
address counter increments by one automatically, allowing
the entire memory contents to be serially read during
sequential Read operation. When the memory address
boundary 32767 is reached, the address counter “rolls
over” to address 0, and the IS24C256 continues to output
data for each ACK received. (Refer to Figure 10. Sequential
Read Operation Starting with a Random Address Read
Diagram.)
Acknowledge (ACK) Polling
The disabling of the inputs can be used to take advantage
of the typical Write cycle time. Once the Stop condition
is issued to indicate the end of the host's Write operation,
the IS24C256 initiates the internal Write cycle. ACK
polling can be initiated immediately. This involves issuing
the Start condition followed by the Slave address for a
Write operation. If the IS24C256 is still busy with the Write
operation, no ACK will be returned. If the IS24C256 has
completed the Write operation, an ACK will be returned
and the host can then proceed with the next Read or Write
operation.
READ OPERATION
Read operations are initiated in the same manner as Write
operations, except that the (R/W) bit of the Slave address
is set to “1”. There are three Read operation options:
current address read, random address read, and sequential
read.
Current Address Read
The IS24C256 contains an internal address counter which
maintains the address of the last byte accessed,
incremented by one. For example, if the previous operation
is either a Read or Write operation addressed to the
address location n, the internal address counter would
increment to address location n+1. When the IS24C256
receives the Device Addressing Byte with a Read operation
(R/W bit set to “1”), it will respond an ACK and transmit the
8-bit data word stored at address location n+1. The
Master should not acknowledge the transfer but should
generate a Stop condition so the IS24C256 discontinues
transmission. If 'n' is the last byte of the memory, then the
data from location '0' will be transmitted. (Refer to
Figure 8. Current Address Read Diagram.)
Integrated Silicon Solution, Inc. — www.issi.com —
京公网安备 11010802033920号
EEWORLD
