DS1921 Просмотр технического описания (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1921

Thermochron iButton

Dallas Semiconductor -> Maxim Integrated 

RTC Alarm Control

DS1921G Thermochron iButton

ALARM REGISTER MASK BITS

(Bit 7 of 207h to 20Ah)

MS MM MH MD

1

1

1

1 Alarm once per second.

0

1

1

1 Alarm when seconds match (once per minute).

0

0

1

1 Alarm when minutes and seconds match (once every hour).

0

0

0

1 Alarm when hours, minutes and seconds match (once every day).

0

0

0

0 Alarm when day, hours, minutes, and seconds match (once every

week).

Temperature Conversion

The DS1921G measures temperatures with a resolution of 0.5°C. Temperature values are represented in a

single byte as an unsigned binary number, which translates 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 will be 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

section Temperature Logging and Histogram) the data in these bins is of limited 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 histogram bins 1 to 61.

With T[7..0] representing the decimal equivalent of a temperature reading, the temperature value is

calculated as

ϑ (°C) = T[7…0] / 2 - 40.0

This equation is valid for converting temperature readings stored in the datalog memory as well as for

data read from the forced temperature conversion readout Register (address 211h).

To specify the high or low temperature alarm thresholds, this equation needs to be resolved to

T[7…0] = 2 * 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).

Temperature Alarm Register Map

ADDR b7

b6

b5

b4

b3

b2

b1

b0

020Bh

Temperature Low Alarm Threshold

020Ch

Temperature High Alarm Threshold

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