DS1923
iButton Hygrochron Temperature/Humidity
Logger 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 DS1923
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 command fails. The device
stops communicating and waits for a reset pulse. If the
password was correct or if passwords 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 pattern 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 CRC command fails. The device
stops communicating and waits for a reset pulse. If the
password was correct or if passwords were not
enabled, the master reads data from the DS1923
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 additional 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 DS1923 transmits 00h or FFh bytes,
respectively. 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 fol-
lowed by the 2 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 shift-
ing 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 DS1923 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