DS1977 Maxim, DS1977 Datasheet - Page 9

• Manufacturer Part Number: DS1977
• Description: The DS1977 is a 32KB EEPROM in a rugged iButton® enclosure
• Manufacturer: Maxim

DS1977
WRITING WITH VERIFICATION
To write data to the DS1977 , the scratchpad has to be used as intermediate storage. First the master issues the
Write Scratchpad command to specify the desired target address, followed by the data to be written to the
scratchpad. Under certain conditions (see Write Scratchpad command) the master will receive an inverted CRC16
of the command, address and data at the end of the write scratchpad command sequence. Knowing this CRC
value, the master can compare it to the value it has calculated itself to decide whether the communication was
successful and proceed to the Copy Scratchpad command. If the master could not receive the CRC16, it has to
send the Read Scratchpad command to read back the scratchpad to verify data integrity. As preamble to the
scratchpad data, the DS1977 repeats the target address TA1 and TA2 and sends the contents of the E/S register.
If the PF flag is set, data did not arrive correctly in the scratchpad or there was a loss of power since data was last
written to the scratchpad. The master does not need to continue reading; it can start a new trial to write data to the
scratchpad. Similarly, a set AA flag together with a cleared PF flag indicates that the Write command was not
recognized by the device. If everything went correctly, both flags are cleared and the ending offset indicates the
address of the last byte written to the scratchpad; the master can continue reading and verifying every data byte.
After the master has verified the data, it has to send the Copy Scratchpad command. This command must be
followed exactly by the data of the three address registers TA1, TA2, and E/S. The master may obtain the contents
of these registers by reading the scratchpad or derive it from the target address and the amount of data to be
written. As soon as the DS1977 has received these bytes correctly and the master has provided an acceptable
password, the DS1977 will copy the scratchpad data to the requested location beginning at the target address.
MEMORY FUNCTION COMMANDS
The “Memory Function Flow Chart” (Figure 7) describes the protocols necessary for accessing the memory and the
special function registers of the DS1977. Examples on how to use these functions to operate the DS1977 are
included at the end of this document, preceding the Electrical Characteristics section. The communication between
master and DS1977 takes place either at standard speed (default, OD = 0) or at Overdrive Speed (OD = 1). If not
explicitly set into the Overdrive mode the DS1977 assumes regular speed.
Write Scratchpad Command [0Fh]
This command is used to specify the target address and to write data to the scratchpad for verification before the
transfer to the EEPROM can be initiated. After issuing the write scratchpad command, the master must first provide
the 2-byte target address, followed by the data to be written to the scratchpad. The data will be written to the
scratchpad starting at the byte offset (T5:T0). The ending offset (E5: E0) will be the byte offset at which the master
stops writing data. Only full data bytes are accepted. If the last data byte is incomplete its content will be ignored
and the partial byte flag PF will be set. When writing to a password address, internal circuitry of the chip will force
the 3 least significant address bits to 0. Only full 8-byte passwords are accepted. The ending offset will be 07 or 0F,
depending on the password(s) to be changed.
When executing the Write Scratchpad command the CRC generator inside the DS1977 (Figure 13) calculates an
inverted CRC over the entire data stream, starting at the command code and ending at the last data byte sent by
the master. This CRC is generated using the CRC16 polynomial by first clearing the CRC generator and then
shifting in the command code (0FH) of the Write Scratchpad command, the Target Addresses TA1 and TA2 as
supplied by the master and all the data bytes. The master may end the Write Scratchpad command at any time.
However, if the ending offset is 3Fh, the master may send 16 read-time slots and will receive the CRC generated
by the DS1977 .
The memory address range of the DS1977 is 0000h to 7FFFh (Figure 5). There is no user-access to the address
range 7FD1h to 7FFFh. If the master sends a target address higher than this, the internal circuitry of the chip will
set the most significant address bit to zero as it is shifted into the internal address register. The Read Scratchpad
command will reveal the target address as it will be used by the DS1977 . The master will identify such address
modifications by comparing the target address read back to the target address transmitted. If the master does not
read the scratchpad, a subsequent copy scratchpad command will not work since the most significant bits of the
target address the master sends will not match the value the DS1977 expects.
Read Scratchpad Command [AAh]
This command is used to verify scratchpad data and target address. After issuing the Read Scratchpad command,
the master begins reading. The first 2 bytes will be the target address. The next byte will be the ending offset/data
status byte (E/S) followed by the scratchpad data beginning at the byte offset (T5:T0), as shown in Figure 6.
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