DS1921H/Z Maxim Integrated Products, DS1921H/Z Datasheet - Page 13

• Manufacturer Part Number: DS1921H/Z
• Description: High Resolution Thermochron Ibutton Range H
• Manufacturer: Maxim Integrated Products

TEMPERATURE ALARM LOGGING
For some applications it may be essential to not only record temperature over time and the temperature
histogram, but also record when exactly the temperature has exceeded a predefined tolerance band and
for how long the temperature stayed outside the desirable range. The DS1921H/Z can log high and low
durations. The tolerance band is specified by means of the Temperature Alarm Threshold Registers,
addresses 20Bh and 20Ch in the register page. One can set a high and a low temperature threshold. See
section Temperature Conversion for the data format the temperature has to be written in. As long as the
temperature values stay within the tolerance band (i.e., are higher than the low threshold and lower than
the high threshold), the DS1921H/Z will not record any temperature alarm. If the temperature during a
mission reaches or exceeds either threshold, the DS1921H/Z will generate an alarm and set either the
Temperature High Flag (THF) or the Temperature Low Flag (TLF) in the Status Register (address 214h).
This way, if the search conditions (address 20Eh) are set accordingly, the master can quickly identify
devices with temperature alarms by means of the Conditional Search function (see ROM Function
Commands). The device also generates a time stamp of when the alarm occurred and begins recording the
duration of the alarming temperature.
Time stamps and durations where the temperature leaves the tolerance band are stored in the address
range 0220h to 027Fh, as shown in Figure 8. This allocation 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
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 occurred. The least
significant byte is stored at the lower address. One address higher than a 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 address 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, another time stamp will be
recorded and its associated counter will increment with each temperature reading outside the tolerance
band. This algorithm is implemented for the low as well as for the high temperature threshold.
0228h to 024Fh
0258h to 027Fh
0224h to 0227h
0254h to 0257h
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
13 of 40
High Alarms 3 to 12
Low Alarms 3 to 12
ALARM EVENT
High Alarm 1
High Alarm 2
Low Alarm 1
Low Alarm 2