Device Operation

READ: The AT49F040 is accessed like an EPROM.

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.

ERASURE: Before a byte can be reprogrammed, the

512K bytes memory array (or 496K bytes if the boot block

featured is used) must be erased. The erased state of the

memory bits is a logical “1". The entire device can be

erased at one time by using a 6-byte software code. The

software chip erase code consists of 6-byte load com-

mands to specific address locations with a specific data

pattern (please refer to the Chip Erase Cycle Waveforms).

After the software chip erase has been initiated, the device

will internally time the erase operation so that no external

clocks are required. The maximum time needed to erase

been enabled, the data in the boot sector will not be

erased.

BYTE PROGRAMMING: Once the memory array is

erased, the device is programmed (to a logical “0") on a

byte-by-byte basis. Please note that a data ”0" cannot be

programmed back to a “1"; only erase operations can con-

vert ”0"s to “1"s. Programming is accomplished via the in-

ternal device command register and is a 4 bus cycle op-

eration (please refer to the Command Definitions table).

The device will automatically generate the required inter-

nal program pulses.

The program cycle has addresses latched on the falling

edge of WE or CE, whichever occurs last, and the data

latched on the rising edge of WE or CE, whichever occurs

(continued)

Block Diagram

The optional 16K bytes boot block section includes a re-

programming write lock out feature to provide data integ-

rity. The boot sector is designed to contain user secure

code, and when the feature is enabled, the boot sector is

permanently protected from being reprogrammed.

Description (Continued)

cle time. The DATA polling feature may also be used to

indicate the end of a program cycle.

BOOT BLOCK PROGRAMMING LOCKOUT: The de-

vice has one designated block that has a programming

lockout feature. This feature prevents programming of

data in the designated block once the feature has been

enabled. The size of the block is 16K bytes. This block,

referred to as the boot block, can contain secure code that

is used to bring up the system. Enabling the lockout fea-

ture will allow the boot code to stay in the device while data

in the rest of the device is updated. This feature does not

have to be activated; the boot block’s usage as a write

protected region is optional to the user. The address range

of the boot block is 00000H to 03FFFH.

Once the feature is enabled, the data in the boot block can

no longer be erased or programmed. Data in the main

memory block can still be changed through the regular

programming method. To activate the lockout feature, a

series of six program commands to specific addresses

with specific data must be performed. Please refer to the

Command Definitions table.

BOOT BLOCK LOCKOUT DETECTION: A software

method is available to determine if programming of the

boot block section is locked out. When the device is in the

software product identification mode (see Software Prod-

uct Identification Entry and Exit sections) a read from ad-

dress location 00002H will show if programming the boot

block is locked out. If the data on I/O0 is low, the boot

block can be programmed; if the data on I/O0 is high, the

program lockout feature has been activated and the block

cannot be programmed. The software product identifica-

tion code should be used to return to standard operation.

4-210

AT49F040