DS1921G Maxim, DS1921G Datasheet - Page 21

• Manufacturer Part Number: DS1921G
• Description: The DS1921G Thermochron® iButton® is a rugged, self-sufficient system that measures temperature and records the result in a protected memory section
• Manufacturer: Maxim

This command is used to verify scratchpad data and
target addresses. After issuing the Read Scratchpad
command, the master begins reading. The first 2 bytes
are the target address. The next byte is the ending off-
set/data status byte (E/S) followed by the scratchpad
data beginning at the byte offset T[4:0], as shown in
Figure 9. Regardless of the actual ending offset, the
master can read data until the end of the scratchpad
after which it receives an inverted CRC-16 of the com-
mand 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 “1”s from the DS1921G until a reset pulse is
issued.
This command is used to copy data from the scratch-
pad to the writable memory sections. Applying a Copy
Scratchpad command to the Sample Rate register can
start a mission provided that several preconditions are
met. See the Mission Start and Logging Process sec-
tion and the flowchart in Figure 11 for details. After issu-
ing 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). If the pattern matches, the AA flag is set and the
copy begins. A pattern of alternating “1”s and “0”s is
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 is ignored. Copy
typically takes 2µs per byte.
______________________________________________________________________________________
Read Scratchpad [AAh]
Copy Scratchpad [55h]
The data to be copied is determined by the three
address registers. The scratchpad data from the begin-
ning offset through the ending offset is copied, starting
at the target address. Anywhere from 1 to 32 bytes can
be copied to memory with this command. The AA flag
remains at logic 1 until it is cleared by the next Write
Scratchpad command. Note that the Copy Scratchpad
command, when applied to the address range 200h to
213h during a mission, ends the mission.
The Read Memory command can be used to read the
entire memory. After issuing the command, the master
must provide the 2-byte target address. After the 2
bytes, the master reads data beginning from the target
address and can continue until the end of memory, at
which point logic “0”s are read. It is important to realize
that the target address registers contain the address
provided. The ending offset/data status byte is unaf-
fected.
The hardware of the DS1921G provides a means to
accomplish error-free writing to the memory section. To
safeguard data in the 1-Wire environment when read-
ing and to simultaneously speed up data transfers, it is
recommended to packetize data into data packets of
the size of one memory page each. Such a packet
would typically store a 16-bit CRC with each page of
data to ensure rapid, error-free data transfers that elim-
inate having to read a page multiple times to verify if
the received data is correct (refer to Application Note
114: 1-Wire File Structure for the recommended file
structure).
Thermochron iButton
Read Memory [F0h]
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