DS1921 DALLAS [Dallas Semiconductor], DS1921 Datasheet - Page 13

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

HISTOGRAM BIN AND TEMPERATURE CROSS-REFERENCE Figure 7
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
TEMPERATURE
READING
FAh
FBh
FCh
FDh
FEh
FFh
00h
01h
02h
03h
04h
05h
06h
07h
08h
F7h
F8h
F9h
DS1921H TEMP.
EQUIV. IN °C
14.500
14.625
14.750
14.875
15.000
15.125
15.250
15.375
15.500
45.375
45.500
45.625
45.750
45.875
46.000
46.125
46.250
46.375
DS1921Z TEMP.
EQUIV. IN °C
13 of 44
25.375
25.500
25.625
25.750
25.875
26.000
26.125
26.250
26.375
-5.500
-5.375
-5.250
-5.125
-5.000
-4.875
-4.750
-4.625
-4.500
HISTOGRAM BIN
NUMBER
61
62
62
62
62
63
63
63
63
0
0
0
0
1
1
1
1
2
HISTOGRAM BIN
87Ch to 87Dh
87Ch to 87Dh
87Ch to 87Dh
87Ch to 87Dh
87Ah to 87Bh
87Eh to 87Fh
87Eh to 87Fh
87Eh to 87Fh
87Eh to 87Fh
800h to 801h
800h to 801h
800h to 801h
800h to 801h
802h to 803h
802h to 803h
802h to 803h
802h to 803h
804h to 805h
ADDRESS
DS1921H/Z