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Read Access Password, Addresses 7FC0h to 7FC7h

The Read Access Password provides access to the function Read Memory. No password applies

when reading from or writing to the scratchpad. If passwords are enabled (EPW = AAh, see

Password Control Register), the 64-bit data pattern that the 1-Wire master has to transmit with the

command flow is compared to the passwords stored in the DS1977 iButton. If the password

matches, access is granted.

Read/Write Access Password, Addresses 7FC8h to 7FCFh

The Read/Write Access Password provides access to the functions Read Memory and Copy

Scratchpad. No password applies when reading from or writing to the scratchpad. If passwords

are enabled (EPW = AAh, see Password Control Register), the 64-bit data pattern that the 1-Wire

master has to transmit with the command flow is compared to the passwords stored in the

DS1977 iButton. If the password matches, access is granted.

Setting up a password is done essentially in the same way as writing data to the user data

memory, only the address is different. Before changing passwords, disable passwords. When

setting up a password, make sure that all 8 bytes of the password are defined. Otherwise the new

password may be unknown. Since they are located in the same memory page, both passwords

can be redefined at the same time. Always verify the scratchpad before sending the copy

scratchpad command. Before enabling passwords, check whether the new password has been

successfully written to the EEPROM chip. See "Verify Password" command for details. After a

new password is successfully copied from the scratchpad to its memory location, erase the

scratchpad by filling it with new data. Otherwise a copy of the password will remain accessible

through the scratchpad until the iButton is disconnected from the 1-Wire line.

Password Control Register

ADDR

b7

b6

b5

b4

b3

b2

b1

b0

7FD0h

EPW

Bit Description

Bit(s)

Definition

EPW: Enable Passwords

b0 to

b7

This byte specifies whether the device will test the validity

of passwords. If the EPW bits form a pattern of

10101010 (AAh), the device will execute these

commands only if the correct password is transmitted. If

the EPW pattern is different from AAh, any data pattern

will be accepted as a valid password. Before enabling

passwords, passwords for read access as well as

read/write access need to be written to the password

registers. The power-on default pattern of EPW is

different from AAh.