5

3354F–FLASH–2/05

AT49F002A(N)(T)

4.5

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 convert “0”s to “1”s. Programming is accomplished via the internal device command register

and is a 4 bus cycle operation (please refer to the Command Definitions table). The device will

automatically generate the required internal 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 first. Programming

is completed after the specified t

indicate the end of a program cycle.

4.6

Boot Block Programming Lockout

The device has one designated block that has a programming lockout feature. This feature pre-

vents 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 feature 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 00000 to 03FFF for the AT49F002A(N) while the address range of the boot

block is 3C000 to 3FFFF for the AT49F002A(N)T.

Once the feature is enabled, the data in the boot block can no longer be erased or programmed

with input voltage levels of 5.5V or less. Data in the main memory block can still be changed

through the regular programming method. To activate the lockout feature, a series of six pro-

gram commands to specific addresses with specific data must be performed. Please refer to the

Command Definitions table.

4.6.1

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 Product Iden-

tification Entry and Exit sections) a read from address location 00002H will show if programming

the boot block is locked out for the AT49F002A(N), and a read from address location 3C002H

will show if programming the boot block is locked out for AT49F002A(N)T. 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 identification

exit code should be used to return to standard operation.

4.6.2

Boot Block Programming Lockout Override

The user can override the boot block programming lockout by taking the RESET pin to 12 volts.

By doing this, protected boot block data can be altered through a chip erase, sector erase or

word programming. When the RESET pin is brought back to TTL levels the boot block program-

ming lockout feature is again active. This feature is not available on the AT49F002AN(T).