DS9101 datasheet(13/42 Pages) MAXIM

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Temperature Conversion

The DS1921G measures temperatures with a resolution

of 0.5°C. Temperature values are represented in a sin-

gle byte as an unsigned binary number, which trans-

lates into a theoretical range of 128°C. The range,

however, has been limited to values from 0000 0000

(00h) through 1111 1010 (FAh). The codes 01h to F9h

are considered valid temperature readings.

If a temperature conversion yields a temperature that is

out of range, it is recorded as 00h (if too low) or FAh (if

too high). Since out-of-range results are accumulated in

histogram bins 0 and 62 (see the

Temperature Logging

and Histogram section), the data in these bins is of lim-

ited value. For this reason the specified temperature

range of the DS1921G is considered to begin at code

04h and end at code F7h, which corresponds to his-

togram bins 1 to 61.

With T[7…0] representing the decimal equivalent of a tem-

perature reading, the temperature value is calculated as

ϑ(°C) = T[7…0]/2 - 40.0

This equation is valid for converting temperature read-

ings stored in the data-log memory as well as for data

read from the Forced Temperature Conversion Readout

To specify the temperature alarm thresholds, this equa-

tion needs to be resolved to

T[7…0] = 2 x

ϑ(°C) + 80.0

A value of 23°C, for example, thus translates into 126

decimal or 7Eh. This corresponds to the binary patterns

0111 1110, which could be written to a Temperature

Alarm register (address 020Bh and 020Ch, respectively).

Sample Rate

The content of the Sample Rate register (address

020Dh) determines how many minutes the temperature

conversions are apart from each other during a mission.

The sample rate can be any value from 1 to 255, coded

as an unsigned 8-bit binary number. If the memory has

been cleared (Status register bit MEMCLR = 1) and a

mission is enabled (Control register bit EM = 0), writing

a nonzero value to the Sample Rate register starts a mis-

sion. For a full description of the correct sequence of

steps to start a temperature-logging mission, see the

Missioning or Mission Example: Prepare and Start a

New Mission sections.

Thermochron iButton

______________________________________________________________________________________

Temperature Alarm Register Map

ADDRESS

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

020Bh

Temperature Low Alarm Threshold

020Ch

Temperature High Alarm Threshold

Sample Rate Register Map

ADDRRESS

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

020Dh

Sample Rate

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