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-
nates bus contention in a system environment. The data
bus will be in a high impedance state when either OE or
CE is HIGH.
Write operations are initiated when both CE and WE are
LOW and OE is HIGH. The X28C512/513 supports both
a CE and WE controlled write cycle. That is, the address
is latched by the falling edge of either CE or WE,
whichever occurs last. Similarly, the data is latched
internally by the rising edge of either CE or WE, which-
ever occurs first. A byte write operation, once initiated,
will automatically continue to completion, typically within
Page Write Operation
The page write feature of the X28C512/513 allows the
entire memory to be written in 2.5 seconds. Page write
allows two to one hundred twenty-eight bytes of data to
be consecutively written to the X28C512/513 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 page address (A7 through A15) for
each subsequent valid write cycle to the part during this
operation must be the same as the initial page address.
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 one hundred twenty-
seven bytes in the same manner as the first byte was
written. Each successive 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 continues to access the device within
the byte load cycle time of 100µs.
Write Operation Status Bits
The X28C512/513 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.
Figure 1. Status Bit Assignment
I/O DP TB 5 4 3 2 1 0
3856 FHD F06
DATA Polling (I/O7)
The X28C512/513 features DATA Polling as a method
to indicate to the host system that the byte write or page
write cycle has completed. DATA Polling allows a simple
bit test operation to determine the status of the X28C512/
513, eliminating additional interrupt inputs or external
hardware. During the internal programming cycle, any
attempt to read the last byte written will produce the
complement of that data on I/O7 (i.e. write data = 0xxx
xxxx, read data = 1xxx xxxx). Once the programming
cycle is complete, I/O7 will reflect true data.
Toggle Bit (I/O6)
The X28C512/513 also provides another method for
determining when the internal write cycle is complete.
During the internal programming cycle, I/O6 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.