X46402 Datasheet, PDF (15/23 Pages) Xicor Inc. – Dual Voltage CPU Supervisor with 64K Password Protected EEPROM

X46402
Note on Read/Write Operations
Preliminary Information
1FFFh
Non- Password Protected
(All of the array to
none of the array)
0000h
Password Protected
(None of the array to
all of the array)
Notes:
Using a “password read” or a “password write” to a non-password protected area is acceptable, because the pass-
word is received and accepted prior to an address transmission. It is assumed that access to non-password pro-
tected areas is uncontrolled, so either method should work.
Using a “no-password read” or a “no-password write” on a password protected area would not work. Trying to
access a password protected area without the password match causes the device to return a NACK after the
address.
A password sequential read that starts in the password protected area can continue into and through the non-pass-
word protected area. It will not “wrap” back to address ’0’.
A no-password sequential read can only start in the non-password protected area and cannot “wrap” back into the
protected area.
CHANGE PASSWORD COMMAND
When changing a password, the Change Password com-
mand is sent to the device. The old password follows.
When the old password is accepted (as indicated by the
ACK Polling Command sequence), the new password is
sent to the device twice, following two bytes of zero. A
stop bit initiates the store of the new password. To be suc-
cessful in the password change operation the first and
second transmission of the new password must match
and there must be exactly 16 password bytes. If this is not
the case, the operation is aborted and the password
remains unchanged.
PASSWORDS
The sequence in Figure 17 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 ignore the
change command and enter into the standby mode.
There are two ways to determine whether the operation
was completed successfully. The Data ACK polling
method can determine if a password has been loaded
correctly, however the data ACK command must be
issued less than 2ms after the stop bit. After this time, it
cannot be determined if the password has been loaded
correctly, without trying the new password. To determine if
the new password has been loaded correctly the data
ACK polling command is issued immediately 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 sub-
sequent data ACK command returns an “ACK”.
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