Data input on the l/O line is latched on the rising edge of
either WE or CE, whichever occurs ﬁrst. Data output on
the l/O line is active whenever both OE and CE are LOW.
Care should be taken to ensure that WE and OE are
never both LOW while CE is LOW.
A read sequence consists of sending a 16-bit address
followed by the reading of data serially. The address is
written by issuing 16 separate write cycles (WE and CE
LOW, OE HIGH) to the part without a read cycle between
the write cycles. The address is sent serially, most signiﬁ-
cant bit ﬁrst, over the I/O line. Note that this sequence is
fully static, with no special timing restrictions, and the
processor is free to perform other tasks on the bus when-
ever the device CE pin is HIGH. Once the 16 address
bits are sent, a byte of data can be read on the I/O line by
issuing 8 separate read cycles (OE and CE LOW, WE
HIGH). At this point, writing a ‘1’ will terminate the read
sequence and enter the low power standby state, other-
wise the device will await further reads in the sequential
The byte address is automatically incremented to the
next higher address after each byte of data is read. The
data stored in the memory at the next address can be
read sequentially by continuing to issue read cycles.
When the highest address in the array is reached, the
address counter rolls over to address $0000 and reading
may be continued indeﬁnitely.
The reset sequence resets the device and sets an inter-
nal write enable latch. A reset sequence can be sent at
any time by performing a read/write “0”/read operation
(see Figs. 1 and 2). This breaks the multiple read or write
cycle sequences that are normally used to read from or
write to the part. The reset sequence can be used at any
time to interrupt or end a sequential read or page load.
As soon as the write “0” cycle is complete, the part is
reset (unless a nonvolatile write cycle is in progress). The
second read cycle in this sequence, and any further read
cycles, will read a HIGH on the l/O pin until a valid read
sequence (which includes the address) is issued. The
reset sequence must be issued at the beginning of both
read and write sequences to be sure the device initiates
these operations properly.
A nonvolatile write sequence consists of sending a reset
sequence, a 16-bit address, up to 64 bytes of data, and
then a special “start nonvolatile write cycle” command
The reset sequence is issued ﬁrst (as described in the
Reset Sequence section) to set an internal write enable
latch. The address is written serially by issuing 16
separate write cycles (WE and CE LOW, OE HIGH) to
the part without any read cycles between the writes. The
address is sent serially, most signiﬁcant bit ﬁrst, on the
l/O pin. Up to 64 bytes of data are written by issuing a
multiple of 8 write cycles. Again, no read cycles are
allowed between writes.
"0" A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0
WHEN ACCESSING: X84256 ARRAY: A15=0
Figure 1. Read Sequence
D7 D6 D5 D4 D3 D2 D1 D0