Datasheet

DS1921G
214h). This way, if the search conditions (address
20Eh) are set accordingly, the master can quickly iden-
tify devices with temperature alarms by means of the
conditional search function (see the ROM Function
Commands
section). The device also generates a time-
stamp of when the alarm occurred and begins record-
ing the duration of the alarming temperature.
Timestamps and durations where the temperature
leaves the tolerance band are stored in the address
range 0220h to 027Fh, as shown in Figure 8. This allo-
cation allows recording 24 individual alarm events and
periods (12 periods for too hot and 12 for too cold). The
date and time of each of these periods can be deter-
mined from the Mission Timestamp register and the
time distance between each temperature reading.
The alarm timestamp is a copy of the Mission Samples
Counter register when the alarm first occurred. The
least significant byte is stored at the lower address.
One address higher than the timestamp, the DS1921G
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 toler-
ance band, the device issues another timestamp at the
next higher alarm location and opens another counter
to record the duration. If the temperature returns to nor-
mal before the counter has reached its limit, the dura-
tion counter of the particular timestamp does not incre-
ment any further. Should the temperature again cross
this threshold, it is recorded at the next available alarm
location. This algorithm is implemented for the low tem-
perature thresholds as well as for the high temperature
threshold.
Missioning
The typical task of the DS1921G i
Button 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, the DS1921G must have its RTC set to a valid time
and date. This reference time can be UTC (also called
GMT, Greenwich Mean Time) or any other time stan-
dard that was chosen for the application. The clock
must be running (EOSC = 0) for at least one second.
Setting an RTC alarm is optional. The memory assigned
to store the alarm timestamps and durations, tempera-
ture histogram, Mission Timestamp, Mission Samples
Counter, Mission Start Delay, and Sample Rate must be
cleared using the Clear Memory 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.
Thermochron iButton
18 ______________________________________________________________________________________
ADDRESS DESCRIPTION ALARM EVENT
0220h Mission Samples Counter, Low Byte
0221h Mission Samples Counter, Center Byte
0222h Mission Samples Counter, High Byte
0223h Alarm Duration Counter
Low Alarm 1
0224h to 0227h Alarm Timestamp and Duration Low Alarm 2
0228h to 024Fh Alarm Timestamp and Durations Low Alarms 3 to 12
0250h Mission Samples Counter, Low Byte
0251h Mission Samples Counter, Center Byte
0252h Mission Samples Counter, High Byte
0253h Alarm Duration Counter
High Alarm 1
0254h to 0257h Alarm Timestamp and Duration High Alarm 2
0258h to 027Fh Alarm Timestamp and Durations High Alarms 3 to 12
Figure 8. Alarm Timestamps and Durations Address Map