DS1922T-F5 MAXIM [Maxim Integrated Products], DS1922T-F5 Datasheet - Page 27

• Manufacturer Part Number: DS1922T-F5
• Description: Temperature Logger iButton With 8kB Datalog Memory
• Manufacturer: MAXIM [Maxim Integrated Products]

DS1922L/DS1922T
MEMORY- AND CONTROL-FUNCTION COMMANDS
The Memory/Control Function Flow Chart (Figure 9) describes the protocols necessary for accessing the memory
and the special function registers of the DS1922L/T. An example on how to use these and other functions to set up
the DS1922L/T for a mission is included at the end of this document, preceding the Electrical Characteristics
section. The communication between master and DS1922L/T takes place either at regular speed (default, OD = 0)
or at Overdrive Speed (OD = 1). If not explicitly set into the Overdrive mode the DS1922L/T assumes regular
speed. Internal memory access during a mission has priority over external access through the 1-Wire interface.
This affects several of the commands described below. See section Memory Access Conflicts for details and
remedies.
WRITE SCRATCHPAD COMMAND [0FH]
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 is written to the scratchpad starting at the byte offset (T4:T0).
The master has to send as many bytes as are needed to reach the Ending Offset of 1Fh. If a data byte is
incomplete, its content is ignored and the partial byte flag PF is set.
When executing the Write Scratchpad command the CRC generator inside the DS1922L/T (see Figure 15)
calculates a CRC of 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. If the ending offset is 11111b, the master may send 16 read time
slots and receive the inverted CRC16 generated by the DS1922L/T.
Note that both register pages are write-protected during a mission. Although the Write Scratchpad command works
normally at any time, the subsequent copy scratchpad to a register page will fail during a mission.
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 is the ending offset/data
status byte (E/S) followed by the scratchpad data beginning at the byte offset (T4:T0), as shown in Figure 8. The
master may continue reading data until the end of the scratchpad after which it receives an inverted CRC16 of the
command code, Target Addresses TA1 and TA2, the E/S byte, and the scratchpad data starting at the target
address. After the CRC is read, the bus master reads logical 1s from the DS1922L/T until a reset pulse is issued.
COPY SCRATCHPAD WITH PASSWORD [99H]
This command is used to copy data from the scratchpad to the writable memory sections. 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 transmit the 64-bit full-access password. If passwords are
enabled and the transmitted password is different from the stored full-access password, the Copy Scratchpad with
Password command will fail. The device will stop communicating and will wait for a reset pulse. If the password
was correct or if passwords were not enabled, the device will test the 3-byte authorization code. If the authorization
code pattern matches, the AA (Authorization Accepted) flag will be set and the copy will begin. A pattern of
alternating 1s and 0s will be transmitted after the data has been copied until the master issues a reset pulse. While
the copy is in progress any attempt to reset the part will be ignored. Copy typically takes 2µs per byte.
The data to be copied is determined by the three address registers. The scratchpad data from the beginning offset
through the ending offset will be copied, starting at the target address. The AA flag will remain at logic 1 until it is
cleared by the next Write Scratchpad command. With suitable password, the copy scratchpad always functions for
the 16 pages of data memory and the 2 pages of calibration memory. While a mission is in progress, write attempts
to the register pages are not successful. The AA bit (Authorization Accepted) remaining at 0 indicates this.
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