X76F640

8

Figure 7. Sequential Read

Data X

S

COMMAND

Read

Password

7

Read

Password

0

S

SDA

Data 0

If ACK, then

OR

ACK POLLING

S

ACK Polling

Repeated

COMMAND

Command

Password Matches

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.

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.

Figure 8. Change Passwords

COMMAND

Old

Password

7

Old

Password

0

S

SDA

New

Password

7

Password

0

New

Password

7

New

Password

0

S

If ACK, then

Two bytes of “0”

ACK POLLING

S

ACK Polling

Repeated

COMMAND

Command

Password Matches

If immediate ACK,

then New Password error

Data ACK

Polling

If immediate NACK,

then New Password OK

followed by ACK after ~5ms

7025 FM 13