DS1921 datasheet(13/44 Pages) DALLAS

13 / 44 page

DS1921H/Z

13 of 44

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

800h to 801h

01h

14.625

-5.375

800h to 801h

02h

14.750

-5.250

800h to 801h

03h

14.875

-5.125

800h to 801h

04h

15.000

-5.000

802h to 803h

05h

15.125

-4.875

802h to 803h

06h

15.250

-4.750

802h to 803h

07h

15.375

-4.625

802h to 803h

08h

15.500

-4.500

804h to 805h

F7h

45.375

25.375

87Ah to 87Bh

F8h

45.500

25.500

87Ch to 87Dh

F9h

45.625

25.625

87Ch to 87Dh

FAh

45.750

25.750

87Ch to 87Dh

FBh

45.875

25.875

87Ch to 87Dh

FCh

46.000

26.000

87Eh to 87Fh

FDh

46.125

26.125

87Eh to 87Fh

FEh

46.250

26.250

87Eh to 87Fh

FFh

46.375

26.375

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

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