DS1977
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Regardless of the actual ending offset the master may continue reading data until the end of the scratchpad after
which it will receive an inverted CRC16 of the command code, Target Addresses TA1 and TA2, the E/S byte, and
the scratchpad data starting at the byte offset, which is determined by the target address. After the CRC is read,
the bus master will read logical 1s from the DS1977 until a reset pulse is issued.
Copy Scratchpad with Password [99h]
This command is used to transfer data from the scratchpad to the memory. After issuing the copy scratchpad
command, the master must provide a 3-byte authorization pattern, which can be obtained by reading the
scratchpad for verification. This pattern must exactly match the data contained in the three address registers (TA1,
TA2, E/S, in that order). Next the master must send a valid full-access password, or, if passwords are not enabled,
8 dummy bytes. Now the master must provide power by bypassing the 1-Wire pullup resistor with an electronic
switch, generating a "strong pullup". If authorization pattern and password are accepted, the AA (Authorization
Accepted) flag will be set and the copy will begin. Copy takes 10ms maximum during which the voltage on the 1-
Wire bus must not fall below 2.8V. After the copy is completed, the master turns off the strong pullup and begins
reading from the 1-Wire. A pattern of alternating 1’s and 0’s will indicate that the copy command was executed
successfully. If the copy command was disturbed due to lack of power or for other reasons (see Figure 7-2, "strong
pullup valid?"), the master will read a constant stream of FFh bytes until it sends a 1-Wire reset pulse. In this case
the destination memory may be incompletely programmed requiring a write scratchpad and copy scratchpad be
repeated to ensure proper programming of the EEPROM. This requires careful consideration when designing
application software that writes to the DS1977 in an intermittent contact environment.
The data to be copied is determined by the three address registers (TA1, TA2, E/S). The scratchpad data from the
beginning offset through the ending offset will be copied to memory, starting at the target address. Anywhere from
1 to 64 bytes may be copied to memory with this command.
Read Memory with Password [69h]
This command is used to read the entire memory, except for the passwords. After issuing the command, the
master must provide the 2-byte target address. Next the master must send a valid read access password, or, if
passwords are not enabled, 8 dummy bytes. Now the master must provide power by bypassing the 1-Wire pullup
resistor with an electronic switch, generating a "strong pullup". If the password was accepted, EEPROM data
beginning at the specified target address and ending at the page boundary will be loaded into the scratchpad
starting at the beginning offset. This transfer takes 5 ms maximum during which the voltage on the 1-Wire bus must
not fall below 2.8V. After the transfer is completed, the master turns off the strong pullup and begins reading from
the 1-Wire. When the end of the memory page (end of scratchpad) is reached, the master will receive an inverted
CRC16 of the command, target address and page data. If the master wants to read more data and the end of the
memory is not yet reached, it again has to activate the strong pullup. This will transfer a full 64-byte page of
memory data to the scratchpad from where the master can read it by issuing read-time slots. This transfer only
takes place if the DS1977 receives enough power through the 1-Wire line (see Figure 7-3, "strong pullup valid?").
The loop of strong pullup and reading 64 bytes can be repeated until the end of the memory is reached, at which
point the master will read logic 1's.
Verify Password [C3h]
This command allows the user to verify whether the process of updating a password was successful, eliminating
the risk of a weak programming of the memory cells that actually store the password. The command allows
verifying one password at a time. After issuing the command code, the master must send the memory address of
the password to be verified. Next the master transmits the password itself and generates a strong pullup to provide
the power for the password comparison. This takes 5ms maximum, during which the voltage on the 1-Wire bus
must not fall below 2.8V. After the comparison is completed, the master turns off the strong pullup and begins
reading from the 1-Wire line. A pattern of alternating 1's and 0's indicates that the verification was successful, i. e.,
the password supplied by the master matches the one stored in the DS1977. If the passwords do not match, the
master will read a constant stream of FFh bytes until it sends a reset pulse.
Before changing a password, first disable the use of passwords. Then using Write Scratchpad, Read Scratchpad
and Copy Scratchpad, write the new password to its respective memory location. Now use Verify Password to
double-check whether the password reads correctly from the EEPROM memory. If the verification is successful, it
is safe to again enable passwords.
