Am28F512 7
BASIC PRINCIPLES
The device uses 100% TTL-level control inputs to
manage the command register. Erase and repro-
gramming operations use a fixed 12.0 V ± 5% high
voltage input.
Read Only Memory
Without high VPP voltage, the device functions as a
read only memory and operates like a standard
EPROM. The control inputs still manage traditional
read, standby, output disable, and Auto select modes.
Command Register
The command register is enabled only when high volt-
age is applied to the VPP pin. The erase and repro-
gramming operations are only accessed via the
register. In addition, two-cycle commands are required
for erase and reprogramming operations. The tradi-
tional read, standby, output disable, and Auto select
modes are available via the register.
The device’s com mand regi ster is wr itten us ing stan-
dard microprocessor write timings. The register con-
trols an internal state machine that manages all device
operations. For system design simplification, the de-
vice is designed to support either WE# or CE# con-
trolled writes. During a system write cycle, addresses
are latched o n the fa lling edge of WE # or CE # w hich-
ever occurs last. Data is latched on the rising edge of
WE# or CE# whichever occur first. To simplify the fol-
lowing discussion, the WE# pin is used as the write
cycle control pin throughout the rest of this text. All
setup and hold times are with respect to the WE# sig-
nal.
Overview of Erase/Pr ogram Operations
Flasherase™ Sequence
A multiple step command sequence is required to
erase the Flash device (a two-cycle Erase command
and repeated one cycle verify commands).
Note: The Flash memory array must be completely
programmed to 0’s prior to erasure. Refer to the
Flashrite™ Programming Algorithm.
1. Erase Setup: Write the Setup Erase command to
the command register .
2. Erase: Wr it e the Erase com mand (sam e as Setu p
Erase command) to the command register again.
The s ec o nd co mm an d in i ti at es t he er a se o pe r at io n.
The system software routines must now time-out
the erase pulse width (10 ms) prior to issuing the
Erase-verify command. An integrated stop timer
pre vents any possibility of overerasure.
3. Erase-Verify: Write the Erase-verify command to
the command register. This command terminates
the erase operation. After the erase operation,
each byte of the array must be verified. Address in-
formation must be supplied with the Erase-verify
command. This command ver ifies the margin and
outpu ts t he addre ss ed b yte i n o rde r to co mpa re th e
array data with FFh data (Byte erased).
After successful data verification the Erase-verify
command is written again with new address infor-
mation. Each by te of the array is sequentially veri-
fied in this manner.
If data of the addressed location is not verified, the
Erase sequence is repeated until the entire array is
successfully verified or the sequence is repeated
1000 times.
Flashrite Programming Sequence
A three step command sequence (a two-cycle Program
command and one cycle Verify command) is required
to pr ogram a byte o f t h e Fla sh ar r a y. Re fer to the Fl a sh-
rite Algorithm.
1. Program Setup: Write the Setup Program com-
mand to the command re gist er.
2. Program: Write the Program command to the com-
mand register with the appropriate Address and
Data. The system software routines must now time-
out the progra m pulse width (10 µs) prio r to issuing
the Program-verify command. An integrated stop
timer pr events any possibility of overprogramming.
3. Program-Verify: Write the Program-verify com-
man d to th e com ma nd re gis t er. Th is c o mm a nd ter -
minates the programming operation. In addition,
this command verifies the margin and outputs the
byte just programmed in order to compare the array
data with the original data programmed. After suc-
cessful data verification, the programming se-
quence is initiated again f or the next byte address to
be programmed.
If data is not verified successfully, the Program se-
quence is repeated until a successful comparison is
verified or the sequence is repeated 25 times.
Data Protection
The devic e is des igned to offer protecti on aga inst acci-
dental erasure or programming caused by spurious
system lev el signals that may exist during power transi-
ti on s. Th e d evice po w er s up in i ts r ea d o nly st at e . Als o,
with its control register architecture, alteration of the
memory contents only occurs after successful comple-
tion of specif ic command s equences.
The device als o incor porates several feat ures to pre-
v ent in advertent write cycles resulting fromVCC power-
up and power-do wn transitions or system noise.
Low VCC Write Inhibit
To avoid initiation of a write cycle during VCC power-up
and p ower-down, th e device locks out write cycles for