Datasheet

DS1921H/Z
13 of 45
HISTOGRAM BIN AND TEMPERATURE CROSS-REFERENCE Figure 7
TEMPERATURE
READING
DS1921H TEMP.
EQUIV. IN °C
DS1921Z TEMP.
EQUIV. IN °C
HISTOGRAM BIN
NUMBER
HISTOGRAM BIN
ADDRESS
00h
14.500
-5.500
0 800h to 801h
01h
14.625
-5.375
0 800h to 801h
02h
14.750
-5.250
0 800h to 801h
03h
14.875
-5.125
0 800h to 801h
04h 15.000 -5.000 1 802h to 803h
05h 15.125 -4.875 1 802h to 803h
06h 15.250 -4.750 1 802h to 803h
07h 15.375 -4.625 1 802h to 803h
08h 15.500 -4.500 2 804h to 805h
F7h 45.375 25.375 61 87Ah to 87Bh
F8h 45.500 25.500 62 87Ch to 87Dh
F9h 45.625 25.625 62 87Ch to 87Dh
FAh 45.750 25.750 62 87Ch to 87Dh
FBh 45.875 25.875 62 87Ch to 87Dh
FCh
46.000
26.000
63 87Eh to 87Fh
FDh
46.125
26.125
63 87Eh to 87Fh
FEh
46.250
26.250
63 87Eh to 87Fh
FFh
46.375
26.375
63 87Eh to 87Fh
Since each data bin is 2 bytes it can increment up to 65535 times. Additional measurements for a bin that
has already reached its maximum value will not be counted; the bin counter will remain at its maximum
value. With the fastest sample rate of one sample every minute, a 2-byte bin is sufficient for up to 45 days
if all temperature readings fall into the same bin.
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