XM28C040 datasheet(3/17 Pages) XICOR

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XM28C040

DEVICE OPERATION

Read

Read operations are initiated by both

OE and CE LOW.

The read operation is terminated by either

CE or OE

returning HIGH. This 2-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

Write operations are initiated when both

CE and WE are

LOW and

OE is HIGH. The XM28C040 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

5ms (see Note 4).

Page Write Operation

The page write feature of the XM28C040 allows the

entire memory to be written in 10 seconds. Page write

allows two to 256 bytes of data to be consecutively

written to the XM28C040 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 (A8 through A18) 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 255 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 100

µs of the falling edge of

the preceding

WE. If a subsequent WE HIGH to LOW

transition is not detected within 100

µs, 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 100

µs.

Write Operation Status Bits

The XM28C040 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.

DATA Polling (I/O7)

Figure 1. Status Bit Assignment

The XM28C040 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

XM28C040, 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. Note: If the XM28C040 is in the protected state and

an illegal write operation is attempted,

DATA Polling will

not operate.

Toggle Bit (I/O6)

The XM28C040 also provides another method for deter-

mining when the internal write cycle is complete. During

the internal programming cycle I/O6 will toggle from “1”

to “0” and “0” to “1” on subsequent attempts to read the

last byte written. When the internal cycle is complete the

toggling will cease and the device will be accessible for

additional read or write operations.

3873 FHD F09

43210

I/O

RESERVED

TOGGLE BIT

DATA POLLING

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