DS1921 DALLAS [Dallas Semiconductor], DS1921 Datasheet - Page 14

• Manufacturer Part Number: DS1921
• Description: High-Resolution Thermochron iButton
• Manufacturer: DALLAS [Dallas Semiconductor]

and periods (12 periods for too hot and 12 for too cold). The date and time of each of these periods can be
determined from the Mission Time Stamp and the time distance between each temperature reading.
ALARM TIME STAMPS AND DURATIONS ADDRESS MAP Figure 8
The alarm time stamp is a copy of the Mission Samples Counter when the alarm first occurred. The least
significant byte is stored at the lower address. One address higher than the time stamp the DS1921H/Z
maintains a 1-byte duration counter that stores the number of samples the temperature was found to be
beyond the threshold. If this counter has reached its limit after 255 consecutive temperature readings and
the temperature has not yet returned to within the tolerance band, the device will issue another time stamp
at the next higher alarm location and open another counter to record the duration. If the temperature
returns to normal before the counter has reached its limit, the duration counter of the particular time
stamp will not increment any further. Should the temperature again cross this threshold, it will be
recorded at the next available alarm location. This algorithm is implemented for the low as well as for the
high temperature threshold.
MISSIONING
The typical task of the DS1921H/Z is recording the temperature of a temperature-sensitive object. Before
the device can perform this function, it needs to be configured. This procedure is called missioning.
First of all, DS1921H/Z needs to have its RTC set to valid time and date. This reference time may be
UTC (also called GMT, Greenwich Mean Time) or any other time standard that was chosen for the
application. The clock must be running (EOSC = 0) for at least one second. Setting a RTC alarm is
optional. The memory assigned to storing alarm time stamps and durations, temperature histogram, as
well as the Mission Time Stamp, Mission Samples Counter, Mission Start Delay, and Sample Rate must
be cleared using the Memory Clear command. In case there were temperature alarms in the previous
mission, the TLF and THF flags need to be cleared manually. To enable the device for a mission, the EM
flag must be set to 0. These are general settings that have to be made regardless of the type of object to be
monitored and the duration of the mission.
Next, the low temperature and high temperature thresholds to specify the temperature tolerance band
must be defined. How to convert a temperature value into the binary code to be written to the threshold
registers is described under Temperature Conversion earlier in this document.
0224h to 0227h
0228h to 024Fh
0254h to 0257h
0258h to 027Fh
ADDRESS
0220h
0221h
0222h
0223h
0250h
0251h
0252h
0253h
Mission Samples Counter Center Byte
Mission Samples Counter Center Byte
Mission Samples Counter High Byte
Mission Samples Counter High Byte
Mission Samples Counter Low Byte
Mission Samples Counter Low Byte
Alarm Time Stamp and Durations
Alarm Time Stamp and Durations
Alarm Time Stamp and Duration
Alarm Time Stamp and Duration
Alarm Duration Counter
Alarm Duration Counter
DESCRIPTION
14 of 44
High Alarms 3 to 12
Low Alarms 3 to 12
ALARM EVENT
High Alarm 1
High Alarm 2
Low Alarm 1
Low Alarm 2
DS1921H/Z