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DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
Counter, and alarm flags must be cleared using the
ETL = 1
TLFS = 0
1000h
ETL = 1
TLFS = 1
1000h
Memory Clear command. To enable the device for a
mission, the ETL bit must be set to 1. These are general
settings that must be made in any case, regardless of
the type of object to be monitored and the duration of
WITH 16-BIT FORMAT,
the mission.
8192
8-BIT ENTRIES
TEMPERATURE
4096
16-BIT ENTRIES
TEMPERATURE
THE MOST SIGNIFICANT
BYTE IS STORED AT THE
LOWER ADDRESS.
If alarm signaling is desired, the temperature alarm low
and high thresholds must be defined. See the
Temperature Conversion section for how to convert a
2FFFh
Figure 7. Temperature Logging
2FFFh
temperature value into the binary code to be written to
the threshold registers. In addition, the temperature
alarm must be enabled for the low and/or high thresh-
old. This makes the device respond to a Conditional
Search command (see the 1-Wire ROM Function
Commands section), provided that an alarming condi-
Data-Log Memory Usage
Once set up for a mission, the DS1922L/DS1922T log
the temperature measurements at equidistant time
points entry after entry in their data-log memory. The
data-log memory can store 8192 entries in 8-bit format
or 4096 entries in 16-bit format (Figure 7). In 16-bit for-
mat, the higher 8 bits of an entry are stored at the lower
address. Knowing the starting time point (Mission
Timestamp) and the interval between temperature mea-
surements, one can reconstruct the time and date of
each measurement.
There are two alternatives to the way the DS1922L/
DS1922T behave 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
Samples Counter in conjunction with the sample rate
and the Mission Timestamp allow 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.
Missioning
The typical task of the DS1922L/DS1922T is recording
temperature. Before the devices can perform this func-
tion, they need to be set up properly. This procedure is
called missioning.
First, the DS1922L/DS1922T must have their RTC set to
a valid time and date. This reference time can be the
local time, or, when used inside of a mobile unit, UTC
(also called GMT, Greenwich Mean Time), or any other
time standard that was agreed upon. The RTC oscillator
must be running (EOSC = 1). The memory assigned to
store the Mission Timestamp, Mission Samples
Maxim Integrated
tion has been encountered.
The setting of the RO bit (rollover enable) and sample
rate depends on the duration of the mission and the
monitoring requirements. If the most recently logged
data is important, the rollover should be enabled (RO =
1). Otherwise, one should estimate the duration of the
mission in minutes and divide the number by 8192
(8-bit format) or 4096 (16-bit format) to calculate the
value of the sample rate (number of minutes between
conversions). For example, if the estimated duration of
a mission is 10 days (= 14400min), the 8192-byte
capacity of the data-log memory would be sufficient to
store a new 8-bit value every 1.8min (110s). If the
DS1922L/DS1922T’s data-log memories are not large
enough to store all readings, one can use several
devices and set the Mission Start Delay to values that
make the second device start logging as soon as the
memory of the first device is full, and so on. The RO bit
must be set to 0 to disable rollover that would otherwise
overwrite the logged data.
After the RO bit and the Mission Start Delay are set, the
sample rate must be written to the Sample Rate regis-
ter. The sample rate can be any value from 1 to 16,383,
coded as an unsigned 14-bit binary number. The
fastest sample rate is one sample per second (EHSS =
1, sample rate = 0001h) and the slowest is one sample
every 273.05hr (EHSS = 0, sample rate = 3FFFh). To
get one sample every 6min, for example, the sample
rate value must be set to 6 (EHSS = 0) or 360 decimal
(equivalent to 0168h at EHSS = 1).
If there is a risk of unauthorized access to the DS1922L/
DS1922T or manipulation of data, one should define
passwords for read access and full access. Before the
passwords become effective, their use must be
enabled. See the Security by Password section for
more details.
27
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