X28C256
DEVICE OPERATION
Read
Write Operation Status Bits
The X28C256 provides the user two write operation
status bits. These can be used to optimize a system
write cycle time. The status bits are mapped onto the
I/O bus as shown in Figure 1.
Read operations are initiated by both OE and CE LOW.
The read operation is terminated by either CE or OE
returning HIGH. This two line control architecture elimi-
natesbuscontentioninasystemenvironment. Thedata
bus will be in a high impedance state when either OE or
CE is HIGH.
Figure 1. Status Bit Assignment
Write
I/O DP TB
5
4
3
2
1
0
Write operations are initiated when bothCE and WE are
LOWandOEisHIGH.TheX28C256supportsbothaCE
and WE controlled write cycle. That is, the address is
latchedbythefallingedgeofeitherCEorWE,whichever
occurslast. Similarly, thedataislatchedinternallybythe
rising edge of either CE or WE, whichever occurs first.
A byte write operation, once initiated, will automatically
continue to completion, typically within 5ms.
RESERVED
TOGGLE BIT
DATA POLLING
3855 FHD F11
DATA Polling (I/O )
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The X28C256 features DATA Polling as a method to
indicate to the host system that the byte write or page
writecyclehascompleted.DATAPollingallowsasimple
bittestoperationtodeterminethestatusoftheX28C256,
eliminating additional interrupt inputs or external hard-
ware. During the internal programming cycle, any at-
tempt to read the last byte written will produce the
Page Write Operation
The page write feature of the X28C256 allows the entire
memory to be written in 2.5 seconds. Page write allows
twotosixty-fourbytesofdatatobeconsecutivelywritten
to the X28C256 prior to the commencement of the
internal programming cycle. The host can fetch data
from another device within the system during a page
write operation (change the source address), but the
complement of that data on I/O (i.e. write data = 0xxx
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xxxx, read data = 1xxx xxxx). Once the programming
cycle is complete, I/O will reflect true data. Note: If the
X28C256 is in the protected state and an illegal write
operation is attempted DATA Polling will not operate.
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page address (A through A ) for each subsequent
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valid write cycle to the part during this operation must be
the same as the initial page address.
Toggle Bit (I/O )
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The page write mode can be initiated during any write
operation. Following the initial byte write cycle, the host
can write an additional one to sixty-three bytes in the
samemannerasthefirstbytewaswritten. Eachsucces-
sive byte load cycle, started by the WE HIGH to LOW
transition, must begin within 100µs of the falling edge of
the preceding WE. If a subsequent WE HIGH to LOW
transition is not detected within 100µs, the internal
automatic programming cycle will commence. There is
no page write window limitation. Effectively the page
write window is infinitely wide, so long as the host
continuestoaccessthedevicewithinthebyteloadcycle
time of 100µs.
The X28C256 also provides another method for deter-
mining when the internal write cycle is complete. During
the internal programming cycle I/O will toggle from
HIGH to LOW and LOW to HIGH on subsequent
attempts to read the device. When the internal cycle is
complete the toggling will cease and the device will be
accessible for additional read or write operations.
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XICOR [ XICOR INC. ]
