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DS1921G
Thermochron iButton Device
Temperature Alarm Register Map
ADDRESS
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
020Bh
020Ch
Temperature Low Alarm Threshold
Temperature High Alarm Threshold
Sample Rate Register Map
ADDRRESS
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
020Dh
Temperature Conversion
The DS1921G measures temperatures with a resolution
of 0.5°C. Temperature values are represented in a sin-
gle byte as an unsigned binary number, which trans-
lates into a theoretical range of 128°C. The range,
however, has been limited to values from 0000 0000
(00h) through 1111 1010 (FAh). The codes 01h to F9h
are considered valid temperature readings.
If a temperature conversion yields a temperature that is
out of range, it is recorded as 00h (if too low) or FAh (if
too high). Since out-of-range results are accumulated in
histogram bins 0 and 62 (see the Temperature Logging
and Histogram section), the data in these bins is of lim-
ited value. For this reason the specified temperature
range of the DS1921G is considered to begin at code
04h and end at code F7h, which corresponds to his-
togram bins 1 to 61.
With T[7…0] representing the decimal equivalent of a tem-
perature reading, the temperature value is calculated as
? (°C) = T[7…0]/2 - 40.0
This equation is valid for converting temperature read-
ings stored in the data-log memory as well as for data
read from the Forced Temperature Conversion Readout
register (address 0211h).
Maxim Integrated
Sample Rate
To specify the temperature alarm thresholds, this equa-
tion needs to be resolved to
T[7…0] = 2 x ? (°C) + 80.0
A value of 23°C, for example, thus translates into 126
decimal or 7Eh. This corresponds to the binary patterns
0111 1110, which could be written to a Temperature
Alarm register (address 020Bh and 020Ch, respectively).
Sample Rate
The content of the Sample Rate register (address
020Dh) determines how many minutes the temperature
conversions are apart from each other during a mission.
The sample rate can be any value from 1 to 255, coded
as an unsigned 8-bit binary number. If the memory has
been cleared (Status register bit MEMCLR = 1) and a
mission is enabled (Control register bit EM = 0), writing
a nonzero value to the Sample Rate register starts a mis-
sion. For a full description of the correct sequence of
steps to start a temperature-logging mission, see the
Missioning or Mission Example: Prepare and Start a
New Mission sections.
13
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