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
Rev. 00D
06/19/07
1
IS24C128A
IS24C256A
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
Rev. 00D
06/18/07
IS24C128A
IS24C256A
PIN CONFIGURATION
8-Pin DIP and SOIC
A0
A1
A2
GND
1
2
3
4
8
7
6
5
VCC
WP
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
SCL
This input clock pin is used to synchronize the data transfer
to and from the device.
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 IS24C32A/64A. 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 values
of A0, A1, and A2 are zero.
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
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.
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.
Integrated Silicon Solution, Inc. — www.issi.com
Rev. 00D
06/18/07
3
IS24C128A
IS24C256A
DEVICE OPERATION
The IS24C128A/256A features a serial communication and
supports a bi-directional 2-wire bus transmission protocol
called I
2
C
TM
.
SCL up to nine times. (For each clock signal transition to
High, the Master checks for a High level on SDA.)
Standby Mode
Power consumption is reduced in standby mode. The
IS24C128A/256A 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
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 receivers. The bus is controlled by
Master device which generates the SCL, controls the bus
access and generates the Stop and Start conditions. The
IS24C128A/256A 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 device
(Fig. 5) address is 8 bits.
The four most significant bits of the Slave device address
are fixed as 1010 for the IS24C128A/256A.
This device has three address bits (A2, A1, and A0),
which allows up to eight IS24C128A/256A 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.
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.
IS24C128A/256A) 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 IS24C128A/256A 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 SDA line must remain stable
whenever the SCL line is high. Any changes in the data
line while the SCL line is high will be interpreted as a
Start or Stop condition.
The state of the SDA line represents valid data after a Start
condition. The SDA 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 IS24C128A/256A 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 the two byte address that are to
be written into the address pointer of the IS24C128A/
256A. After receiving another ACK from the Slave, the
Master device transmits the data byte to be written into the
address memory location. The IS24C128A/256A
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 IS24C128A/256A 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
4
Integrated Silicon Solution, Inc. — www.issi.com
Rev. 00D
06/18/07
IS24C128A
IS24C256A
Page Write
The IS24C128A/256A 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 IS24C128A/256A 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 a byte
address is incremented from the last byte of a page, it
returns to the first byte of that page. 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 IS24C128A/256A in a single
Write cycle. All inputs are disabled until completion of the
internal Write cycle.
should generate a Stop condition so the IS24C128A/256A
discontinues transmission. If 'n' is the last byte of the
memory, the data from location '0' will be transmitted. (Refer
to Figure 8. Current Address Read Diagram.)
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 IS24C128A/256A
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 IS24C128A/256A 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
IS24C128A/256A sends the initial byte sequence, the
Master device now responds with an ACK indicating it
requires additional data from the IS24C128A/256A. The
IS24C128A/256A 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 of
16383 or 32767 (depending on the device) is reached, the
address counter “rolls over” to address 0, and the IS24C128A/
256A 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 IS24C128A/256A 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 IS24C128A/256A is still busy with the
Write operation, no ACK will be returned. If the IS24C128A/
256A 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 IS24C128A/256A 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 IS24C128A/
256A receives the Slave 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
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