(continued)

To allow for simple in-system reprogrammability, the

AT29C040A does not require high input voltages for pro-

gramming. Five-volt-only commands determine the opera-

tion of the device. Reading data out of the device is similar

t o rea ding from an EPROM. Reprogramming the

AT29C040A is performed on a sector basis; 256-bytes of

data are loaded into the device and then simultaneously

programmed.

Description (Continued)

Device Operation

READ:

The AT29C040A 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.

BYTE LOAD:

Byte loads are used to enter the 256-

bytes of a sector to be programmed or the software codes

for data protection. A byte load is performed by applying a

low pulse on the WE or CE input with CE or WE low (re-

spectively) and OE high. The address is latched on the

falling edge of CE or WE, whichever occurs last. The data

is latched by the first rising edge of CE or WE.

PROGRAM:

The device is reprogrammed on a sector

basis. If a byte of data within a sector is to be changed,

data for the entire sector must be loaded into the device.

Any byte that is not loaded during the programming of its

sector will be erased to read FFH. Once the bytes of a

sector are loaded into the device, they are simultaneously

programmed during the internal programming period. Af-

ter the first data byte has been loaded into the device, suc-

cessive bytes are entered in the same manner. Each new

byte to be programmed must have its high to low transition

on WE (or CE) within 150

µs of the low to high transition of

WE (or CE) of the preceding byte. If a high to low transition

is not detected within 150

µs of the last low to high transi-

tion, the load period will end and the internal programming

period will start. A8 to A18 specify the sector address. The

sector address must be valid during each high to low tran-

Block Diagram

During a reprogram cycle, the address locations and 256-

bytes of data are internally latched, freeing the address

and data bus for other operations. Following the initiation

of a program cycle, the device will automatically erase the

sector and then program the latched data using an internal

control timer. The end of a program cycle can be detected

by DATA polling of I/O7. Once the end of a program cycle

has been detected, a new access for a read or program

can begin.

sition of WE (or CE). A0 to A7 specify the byte address

within the sector. The bytes may be loaded in any order;

sequential loading is not required. Once a programming

read operation will effectively be a polling operation.

SOFTWARE DATA PROTECTION:

A software control-

led data protection feature is available on the AT29C040A.

Once the software protection is enabled a software algo-

rithm must be issued to the device before a program may

be performed. The software protection feature may be en-

abled or disabled by the user; when shipped from Atmel,

the software data protection feature is disabled. To enable

the software data protection, a series of three program

commands to specific addresses with specific data must

be performed. After the software data protection is en-

abled the same three program commands must begin

each program cycle in order for the programs to occur. All

software program commands must obey the sector pro-

gram timing specifications. The SDP feature protects all

sectors, not just a single sector. Once set, the software

data protection feature remains active unless its disable

command is issued. Power transitions will not reset the

software data protection feature, however the software

feature will guard against inadvertent program cycles dur-

ing power transitions.

After setting SDP, any attempt to write to the device with-

out the three-byte command sequence will start the inter-

nal write timers. No data will be written to the device; how-

tively be a polling operation.

2

AT29C040A