Datasheet

For the temperature histogram, the DS1921G provides
63 bins that begin at memory address 0800h. Each bin
consists of a 16-bit, nonrolling-over binary counter that
is incremented each time a temperature value acquired
during a mission falls into the range of the bin. The least
significant byte of each bin is stored at the lower
address. Bin 0 begins at memory address 0800h, bin 1
at 0802h, and so on up to 087Ch for bin 62, as shown
in Figure 7. The number of the bin to be updated after a
temperature conversion is determined by cutting off the
two least significant bits of the binary temperature
value. Out-of-range values are range locked and count-
ed as 00h or FAh.
Since each data bin is 2 bytes, it can increment up to
65,535 times. Additional measurements for a bin that
has already reached its maximum value are not count-
ed; the bin counter remains 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 tempera-
ture readings fall into the same bin.
Temperature Alarm Logging
For some applications it is essential to not only record
temperature over time and the temperature histogram,
but also record when exactly the temperature exceed-
ed a predefined tolerance band and for how long the
temperature stayed outside the desirable range. The
DS1921G can log high and low durations. The toler-
ance band is specified by means of the Temperature
Alarm Threshold registers, addresses 20Bh and 20Ch
in the register page. One can set a high temperature
and low temperature threshold. See the Temperature
Conversion section 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
DS1921G does not record any temperature alarm. If the
temperature during a mission reaches or exceeds
either threshold, the DS1921G generates an alarm and
sets either the temperature high flag (THF) or the tem-
perature low flag (TLF) in the Status register (address
DS1921G
Thermochron iButton
______________________________________________________________________________________ 17
TEMPERATURE READING
TEMPERATURE
EQUIVALENT IN °C
HISTOGRAM BIN NUMBER HISTOGRAM BIN ADDRESS
00h -40.0 or lower 0 800h to 801h
01h -39.5 0 800h to 801h
02h -39.0 0 800h to 801h
03h -38.5 0 800h to 801h
04h -38.0 1 802h to 803h
05h -37.5 1 802h to 803h
06h -37.0 1 802h to 803h
07h -36.5 1 802h to 803h
08h -36.0 2 804h to 805h
… … … …
F3h +81.5 60 878h to 879h
F4h +82.0 61 87Ah to 87Bh
F5h +82.5 61 87Ah to 87Bh
F6h +83.0 61 87Ah to 87Bh
F7h +83.5 61 87Ah to 87Bh
F8h +84.0 62 87Ch to 87Dh
F9h +84.5 62 87Ch to 87Dh
FAh +85.0 or higher 62 87Ch to 87Dh
Figure 7. Histogram Bin and Temperature Cross-Reference