X76F640 Datasheet, PDF (8/17 Pages) Xicor Inc. – Secure SerialFlash

X76F640
Figure 7. Sequential Read
SDA _S
COMMAND
If ACK, then
Password Matches
ACK POLLING
COMMAND
S
Read
Password
7
Read
Password
0
Wait tWC
OR
Repeated
ACK Polling
Command
Data 0
Data X
S
7025 FM 12
PASSWORDS
The sequence in Figure 8 shows how to change (pro-
gram) the passwords. The programming of passwords is
done twice prior to the nonvolatile write cycle in order to verify
that the new password is consistent. After the eight
bytes are entered in the second pass, a comparison takes
place. A mismatch will cause the part to reset and
enter into the standby mode.
Data ACK polling can be used to determine if a password has
been loaded correctly, however the data ACK com-
mand must be issued less than 2ms after the stop bit.
Figure 8. Change Passwords
SDA _S
COMMAND
Old
Password
7
After this time, it cannot be determined if the password has
been loaded correctly, without trying the new pass-
word. To determine if the new password has been loaded
correctly the data ACK polling command is issued imme-
diately following the stop bit. If it returns an ACK, then the two
passes of the new password entry do not match. If it
returns a "no ACK" then the passwords match and a high
voltage cycle is in progress. The high voltage cycle is
complete when a subsequent data ACK command
returns an "ACK".
There is no way to read any of the passwords.
Old
Password
0
Wait tWC OR
Repeated
ACK Polling
Command
If ACK, then
Password Matches
ACK POLLING
COMMAND
S
Two bytes of “0”
New
Password
7
New
Password
0
New
Password
7
8
Password
0
Data ACK
Polling
If immediate ACK,
S
then New Password error
If immediate NACK,
followed by ACK after ~5ms
then New Password OK
7025 FM 13