DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
Read Scratchpad [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
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 8. The master can continue reading data until
the end of the scratchpad after which it receives an
inverted CRC-16 of the command code, target
addresses TA1 and TA2, the E/S byte, and the scratch-
pad data starting at the target address. After the CRC
is read, the bus master reads logic 1s from the
DS1922L/DS1922T until a reset pulse is issued.
Copy Scratchpad with Password [99h]
This command is used to copy data from the scratch-
pad 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 pass-
word. If passwords are enabled and the transmitted
password is different from the stored full-access pass-
word, the Copy Scratchpad with Password command
fails. The device stops communicating and waits for a
reset pulse. If the password was correct or if pass-
words were not enabled, the device tests the 3-byte
authorization code. If the authorization code pattern
matches, the AA flag is set and the copy begins. A pat-
tern of alternating 1s and 0s 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.
The data to be copied is determined by the three
address registers. The scratchpad data from the begin-
ning offset through the ending offset are copied, start-
ing at the target address. The AA flag remains at logic
1 until it is cleared by the next Write Scratchpad com-
mand. 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
30
progress, write attempts to the register pages are not
successful. The AA bit remaining at 0 indicates this.
Read Memory with Password and
CRC [69h]
The Read Memory with CRC command is the general
function to read from the device. This command gener-
ates and transmits a 16-bit CRC following the last data
byte of a memory page.
After having sent the command code of the Read
Memory with CRC command, the bus master sends a
2-byte address that indicates a starting byte location.
Next, the master must transmit one of the 64-bit pass-
words. If passwords are enabled and the transmitted
password does not match one of the stored passwords,
the Read Memory with Password and CRC command
fails. The device stops communicating and waits for a
reset pulse. If the password was correct or if pass-
words were not enabled, the master reads data from
the DS1922L/DS1922T beginning from the starting
address and continuing until the end of a 32-byte page
is reached. At that point the bus master sends 16 addi-
tional read-data time slots and receives the inverted 16-
bit CRC. With subsequent read-data time slots the
master receives data starting at the beginning of the
next memory page followed again by the CRC for that
page. This sequence continues until the bus master
resets the device. When trying to read the passwords
or memory areas that are marked as “reserved,” the
DS1922L/DS1922T transmit 00h or FFh bytes, respec-
tively. The CRC at the end of a 32-byte memory page is
based on the data as it was transmitted.
With the initial pass through the read memory with CRC
flow, the 16-bit CRC value is the result of shifting the
command byte into the cleared CRC generator followed
by the two address bytes and the contents of the data
memory. Subsequent passes through the read memory
with CRC flow generate a 16-bit CRC that is the result
of clearing the CRC generator and then shifting in the
contents of the data memory page. After the 16-bit CRC
of the last page is read, the bus master receives logic
1s from the DS1922L/DS1922T until a reset pulse is
issued. The Read Memory with CRC command
sequence can be ended at any point by issuing a reset
pulse.
Maxim Integrated