4 Mbit SuperFlash EEPROM
SST28SF040A / SST28VF040A
Data Sheet
The Read operation of the SST28SF/VF040A are con-
trolled by OE# and CE# at logic low. When CE # is high,
the chip is deselected and only standby power will be con-
sumed. OE# is the output control and is used to gate data
from the output pins. The data bus is in high impedance
state when CE# or OE# are high.
rising edge of OE# or CE#, whichever occurs first. A similar
seven read sequence of 1823H, 1820H, 1822H, 0418H,
041BH, 0419H, 040AH will protect the device. Also refer to
Figures 10 and 11 for the 7 read cycle sequence Software
Data Protection. The I/O pins can be in any state (i.e., high,
low, or tri-state).
Read-ID operation
Write Operation Status Detection
The Read-ID operation is initiated by writing a single com-
mand (90H). A read of address 0000H will output the man-
ufacturer’s ID (BFH). A read of address 0001H will output
the device ID (04H). Any other valid command will termi-
nate this operation.
The SST28SF/VF040A provide three means to detect the
completion of a Write operation, in order to optimize the
system Write operation. The end of a Write operation
(Erase or Program) can be detected by three means: 1)
monitoring the Data# Polling bit, 2) monitoring the Toggle
bit, or 3) by two successive reads of the same data. These
three detection mechanisms are described below.
Data Protection
The actual completion of the nonvolatile Write is asynchro-
nous with the system; therefore, either a Data# Polling or
Toggle Bit read may be simultaneous with the completion
of the Write cycle. If this occurs, the system may possibly
get an erroneous result, i.e., valid data may appear to con-
flict with the DQ used. In order to prevent spurious rejec-
tion, if an erroneous result occurs, the software routine
should include a loop to read the accessed location an
additional two (2) times. If both reads are valid, then the
device has completed the Write cycle, otherwise the rejec-
tion is valid.
In order to protect the integrity of nonvolatile data storage,
the SST28SF/VF040A provide both
hardware and software features to prevent inadvertent
writes to the device, for example, during system power-up
or power-down. Such provisions are described below.
Hardware Data Protection
The SST28SF/VF040A are designed with hardware fea-
tures to prevent inadvertent writes. This is done in the fol-
lowing ways:
1. Write Cycle Inhibit Mode: OE# low, CE#, or WE#
high will inhibit the Write operation.
Data# Polling (DQ7)
The SST28SF/VF040A feature Data# Polling to indicate
the Write operation status. During a Write operation, any
attempt to read the last byte loaded during the byte-load
cycle will receive the complement of the true data on DQ7.
Once the Write cycle is completed, DQ7 will show true
data. The device is then ready for the next operation. See
Figure 12 for Data# Polling timing waveforms. In order for
Data# Polling to function correctly, the byte being polled
must be erased prior to programming.
2. Noise/Glitch Protection: A WE# pulse width of less
than 5 ns will not initiate a Write cycle.
3. VDD Power Up/Down Detection: The Write opera-
tion is inhibited when VDD is less than 2.0V.
4. After power-up, the device is in the Read mode
and the device is in the Software Data Protect
state.
Software Data Protection (SDP)
Toggle Bit (DQ6)
The SST28SF/VF040A have software methods to further
prevent inadvertent writes. In order to perform an Erase or
Program operation, a two-step command sequence con-
sisting of a set-up command followed by an execute com-
mand avoids inadvertent erasing and programming of the
device.
An alternative means for determining the Write operation
status is by monitoring the Toggle Bit, DQ6. During a Write
operation, consecutive attempts to read data from the
device will result in DQ6 toggling between logic 0 (low) and
logic 1 (high). When the Write cycle is completed, the tog-
gling will stop. The device is then ready for the next opera-
tion. See Figure 13 for Toggle Bit timing waveforms.
The SST28SF/VF040A will default to Software Data Pro-
tection after power up. A sequence of seven consecutive
reads at specific addresses will unprotect the device The
address sequence is 1823H, 1820H, 1822H, 0418H,
041BH, 0419H, 041AH. The address bus is latched on the
©2001 Silicon Storage Technology, Inc.
S71077-04-000 6/01 310
3
SST [ SILICON STORAGE TECHNOLOGY, INC ]
