AT28C256
Features
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Fast Read Access Time - 150 ns
Automatic Page Write Operation
Internal Address and Data Latches for 64-Bytes
Internal Control Timer
Fast Write Cycle Times
Page Write Cycle Time: 3 ms or 10 ms Maximum
1 to 64-Byte Page Write Operation
Low Power Dissipation
50 mA Active Current
200
µA
CMOS Standby Current
Hardware and Software Data Protection
DATA Polling for End of Write Detection
High Reliability CMOS Technology
Endurance: 10
4
or 10
5
Cycles
Data Retention: 10 Years
Single 5V
±
10% Supply
CMOS and TTL Compatible Inputs and Outputs
JEDEC Approved Byte-Wide Pinout
Full Military, Commercial, and Industrial Temperature Ranges
256K (32K x 8)
Paged
CMOS
E
2
PROM
Description
The AT28C256 is a high-performance Electrically Erasable and Programmable Read
Only Memory. Its 256K of memory is organized as 32,768 words by 8 bits. Manufac-
tured with Atmel’s advanced nonvolatile CMOS technology, the device offers access
times to 150 ns with power dissipation of just 440 mW. When the device is deselected,
the CMOS standby current is less than 200
µA.
(continued)
Pin Configurations
Pin Name
A0 - A14
CE
OE
WE
I/O0 - I/O7
NC
DC
Function
Addresses
Chip Enable
Output Enable
Write Enable
Data Inputs/Outputs
No Connect
Don’t Connect
TSOP
Top View
AT28C256
PGA
Top View
LCC, PLCC
Top View
CERDIP, PDIP,
FLATPACK, SOIC
Top View
Note: PLCC package pins 1 and
17 are DON’T CONNECT.
0006F
2-217
Description
(Continued)
The AT28C256 is accessed like a Static RAM for the read
or write cycle without the need for external components.
The device contains a 64-byte page register to allow writ-
ing of up to 64-bytes simultaneously. During a write cycle,
the addresses and 1 to 64-bytes of data are internally
latched, freeing the address and data bus for other opera-
tions. Following the initiation of a write cycle, the device
will automatically write the latched data using an internal
control timer. The end of a write cycle can be detected by
DATA POLLING of I/O7. Once the end of a write cycle has
been detected a new access for a read or write can begin.
Atmel’s 28C256 has additional features to ensure high
quality and manufacturability. The device utilizes internal
error correction for extended endurance and improved
data retention characteristics. An optional software data
protection mechanism is available to guard against inad-
vertent writes. The device also includes an extra 64-bytes
of E
2
PROM for device identification or tracking.
Block Diagram
Absolute Maximum Ratings*
Temperature Under Bias................. -55°C to +125°C
Storage Temperature...................... -65°C to +150°C
All Input Voltages
(including NC Pins)
with Respect to Ground ................... -0.6V to +6.25V
All Output Voltages
with Respect to Ground .............-0.6V to V
CC
+ 0.6V
Voltage on OE and A9
with Respect to Ground ................... -0.6V to +13.5V
*NOTICE: Stresses beyond those listed under “Absolute Maxi-
mum Ratings” may cause permanent damage to the device.
This is a stress rating only and functional operation of the
device at these or any other conditions beyond those indi-
cated in the operational sections of this specification is not
implied. Exposure to absolute maximum rating conditions
for extended periods may affect device reliability.
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AT28C256
AT28C256
Device Operation
READ:
The AT28C256 is accessed like a Static RAM.
When CE and OE are low and WE is high, the data stored
at the memory location determined by the address pins is
asserted on the outputs. The outputs are put in the high
impedance state when either CE or OE is high. This dual-
line control gives designers flexibility in preventing bus
contention in their system.
BYTE WRITE:
A low pulse on the WE or CE input with CE
or WE low (respectively) and OE high initiates a write cy-
cle. The address is latched on the falling edge of CE or
WE, whichever occurs last. The data is latched by the first
rising edge of CE or WE. Once a byte write has been
started it will automatically time itself to completion. Once
a programming operation has been initiated and for the
duration of t
WC
, a read operation will effectively be a poll-
ing operation.
PAGE WRITE:
The page write operation of the AT28C256
allows 1 to 64-bytes of data to be written into the device
during a single internal programming period. A page write
operation is initiated in the same manner as a byte write;
the first byte written can then be followed by 1 to 63 addi-
tional bytes. Each successive byte must be written within
150
µs
(t
BLC
) of the previous byte. If the t
BLC
limit is ex-
ceeded the AT28C256 will cease accepting data and com-
mence the internal programming operation. All bytes dur-
ing a page write operation must reside on the same page
as defined by the state of the A6 - A14 inputs. For each
WE high to low transition during the page write operation,
A6 - A14 must be the same.
The A0 to A5 inputs are used to specify which bytes within
the page are to be written. The bytes may be loaded in any
order and may be altered within the same load period.
Only bytes which are specified for writing will be written;
unnecessary cycling of other bytes within the page does
not occur.
DATA POLLING:
The AT28C256 features DATA Polling
to indicate the end of a write cycle. During a byte or page
write cycle an attempted read of the last byte written will
result in the complement of the written data to be pre-
sented on I/O7. Once the write cycle has been completed,
true data is valid on all outputs, and the next write cycle
may begin. DATA Polling may begin at anytime during the
write cycle.
TOGGLE BIT:
In addition to DATA Polling the AT28C256
provides another method for determining the end of a write
cycle. During the write operation, successive attempts to
read data from the device will result in I/O6 toggling be-
tween one and zero. Once the write has completed, I/O6
will stop toggling and valid data will be read. Reading the
toggle bit may begin at any time during the write cycle.
DATA PROTECTION:
If precautions are not taken, inad-
vertent writes may occur during transitions of the host sys-
tem power supply. Atmel has incorporated both hardware
and software features that will protect the memory against
inadvertent writes.
HARDWARE PROTECTION:
Hardware features protect
against inadvertent writes to the AT28C256 in the follow-
ing ways: (a) V
CC
sense - if V
CC
is below 3.8V (typical) the
write function is inhibited; (b) V
CC
power-on delay - once
V
CC
has reached 3.8V the device will automatically time
out 5 ms (typical) before allowing a write: (c) write inhibit -
holding any one of OE low, CE high or WE high inhibits
write cycles; (d) noise filter - pulses of less than 15 ns (typi-
cal) on the WE or CE inputs will not initiate a write cycle.
SOFTWARE DATA PROTECTION:
A software controlled
data protection feature has been implemented on the
AT28C256. When enabled, the software data protection
(SDP), will prevent inadvertent writes. The SDP feature
may be enabled or disabled by the user; the AT28C256 is
shipped from Atmel with SDP disabled.
SDP is enabled by the host system issuing a series of
three write commands; three specific bytes of data are
written to three specific addresses (refer to Software Data
Protection Algorithm). After writing the 3-byte command
sequence and after t
WC
the entire AT28C256 will be pro-
tected against inadvertent write operations. It should be
noted, that once protected the host may still perform a
byte or page write to the AT28C256. This is done by pre-
ceding the data to be written by the same 3-byte command
sequence used to enable SDP.
Once set, SDP will remain active unless the disable com-
mand sequence is issued. Power transitions do not dis-
able SDP and SDP will protect the AT28C256 during
power-up and power-down conditions. All command se-
quences must conform to the page write timing specifica-
tions. The data in the enable and disable command se-
quences is not written to the device and the memory ad-
dresses used in the sequence may be written with data in
either a byte or page write operation.
After setting SDP, any attempt to write to the device with-
out the 3-byte command sequence will start the internal
write timers. No data will be written to the device; however,
for the duration of t
WC
, read operations will effectively be
polling operations.
(continued)
2-219
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