Rainbow Electronics AT29C257 - Manual (Page 2)

Block Diagram
Device Operation
READ: The AT29C257 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 whenever CE or OE is high. This dual-
line control gives designers flexibility in preventing bus
contention.
BYTE LOAD: A byte load is performed by applying a
low pulse on the WE or CE input with CE or WE low (re-
spectively) and OE high. 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. Byte loads
are used to enter the 64-bytes of a page to be pro-
grammed or the software codes for data protection and
chip erasure.
PROGRAM: The device is reprogrammed on a page
basis. If a byte of data within a page is to be changed, data
for the entire page must be loaded into the device. Any
byte that is not loaded during the programming of its page
will be indeterminate. Once the bytes of a page are loaded
into the device, they are simultaneously programmed dur-
ing the internal programming period. After the first data
byte has been loaded into the device, successive bytes
are entered in the same manner. Each new byte to be pro-
grammed must have its high to low transition on WE (or
CE) within 150
µ
s of the low to high transition of WE (or
CE) of the preceding byte. If a high to low transition is not
detected within 150
µ
s of the last low to high transition, the
load period will end and the internal programming period
will start. A6 to A14 specify the page address. The page
address must be valid during each high to low transition of
WE (or CE). A0 to A5 specify the byte address within the
page. The bytes may be loaded in any order; sequential
loading is not required. Once a programming operation
has been initiated, and for the duration of t
WC
, a read op-
eration will effectively be a polling operation. (continued)
SOFTWARE DATA PROTECTION: A software control-
led data protection feature is available on the AT29C257.
Once the software protection is enabled a software algo-
rithm must be issued to the device before a program may
be performed. The software protection feature may be en-
abled or disabled by the user; when shipped from Atmel,
the software data protection feature is disabled. To enable
the software data protection, a series of three program
commands to specific addresses with specific data must
be performed. After the software data protection is en-
abled the same three program commands must begin
each program cycle in order for the programs to occur. All
software program commands must obey the page pro-
gram timing specifications. Once set, the software data
protection feature remains active unless its disable com-
mand is issued. Power transitions will not reset the soft-
ware data protection feature, however the software fea-
ture will guard against inadvertent program cycles during
power transitions.
Once set, software data protection will remain active un-
less the disable command sequence is issued.
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
, a read operation will effectively be
a polling operation.
After the software data protection’s 3-byte command code
is given, a byte load is performed by applying a low pulse
on the WE or CE input with CE or WE low (respectively)
and OE high. 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. The 64-bytes of data
must be loaded into each sector by the same procedure as
outlined in the program section under device operation.
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AT29C257