
DS1923: Hygrochron Temperature/Humidity Logger iButton with 8kB Data Log Memory
The Full Access Password needs to be transmitted exactly in the sequence FP0, FP1… FP62, FP63. It affects the
functions “Read Memory with CRC”, “Copy Scratchpad”, “Clear Memory”, “Start Mission”, and “Stop Mission”. The
DS1923 executes the command only if the password transmitted by the master was correct or if password
checking is not enabled
Due to the special behavior of the write access logic, the Password Control Register and both passwords must be
written at the same time. When setting up new passwords, always verify (read back) the scratchpad before sending
the copy scratchpad command. After a new password is successfully copied from the scratchpad to its memory
location, erase the scratchpad by filling it with new data (write scratchpad command). Otherwise a copy of the
passwords will remain in the scratchpad for public read access.
DATA LOG MEMORY USAGE
Once setup for a mission, the DS1923 logs the temperature measurements and/or humidity at equidistant time
points entry after entry in its data log memory. The data log memory is able to store 8192 entries in 8-bit format or
4096 entries in 16-bit format (Figure 7A). If temperature as well as humidity is logged, both in the same format, the
memory is split into two equal sections that can store 4096 8-bit entries or 2048 16-bit entries (Figure 7B). If the
device is set up to log data in different formats, e. g., temperature in 8-bit and humidity in 16-bit format, the memory
is split into blocks of different size, accommodating 2560 entries for either data source (Figure 7C). In this case, the
upper 256 bytes are not used. In 16-bit format, the higher 8 bits of an entry are stored at the lower address.
Knowing the starting time point (Mission Time Stamp) and the interval between temperature measurements one
can reconstruct the time and date of each measurement.
There are two alternatives to the way the DS1923 behaves after the data log memory is filled with data. The user
can program the device to either stop any further recording (disable “rollover”) or overwrite the previously recorded
data (enable “rollover”), one entry at a time, starting again at the beginning of the respective memory section. The
contents of the Mission Sample Counter in conjunction with the sample rate and the Mission Time Stamp then
allows reconstructing the time points of all values stored in the data log memory. This gives the exact history over
time for the most recent measurements taken. Earlier measurements cannot be reconstructed.
Figure 7A. ONE CHANNEL LOGGING
ETL = 1; EHL = 0 or
ETL = 0; EHL = 1
TLFS = HLFS = 0
TLFS = HLFS = 1
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8192
8-bit entries
Temperature
or
Humidity data
4096
16-bit entries
Temperature
or
Humidity data
1000h
2FFFh
1000h
2FFFh
ETL = 1; EHL = 0 or
ETL = 0; EHL = 1
With 16-bit format,
the most-significant
byte is stored at the
lower address.