DS1921 datasheet(8/44 Pages) DALLAS

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DS1921H/Z

8 of 44

RTC Alarm Control

ALARM REGISTER MASK BITS

(Bit 7 of 207h to 20Ah)

Alarm once per second.

Alarm when seconds match (once per minute).

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

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

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

week).

TEMPERATURE CONVERSION

The DS1921H and DS1921Z measure temperatures with a resolution of 1/8

th of a degree Celsius.

Temperature values are represented in a single byte as an unsigned binary number, which translates into a

range of 32°C. The possible values are 0000 0000 (00h) through 1111 1111 (FFh). The codes 01h to FEh

are considered valid temperature readings. Since the DS1921H and DS1921Z have different starting

temperatures, the meaning of a binary temperature code depends on the device.

If a temperature conversion yields a temperature that is out-of-range, it will be recorded as 00h (if too

low) or FFh (if too high). Since out-of-range results are accumulated in histogram bins 0 and 63 the data

in these bins is of limited value (see the Temperature Logging and Histogram section). For this reason the

specified temperature range of the DS1921H and DS1921Z is considered to begin at code 04h and end at

code FBh, which corresponds to histogram bins 1 to 62.

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

calculated as

ϑ (°C) = T[7…0] / 8 + 14.500 (DS1921H)

ϑ (°C) = T[7…0] / 8 - 5.500 (DS1921Z)

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] = 8 *

ϑ (°C) -116 (DS1921H)

T[7…0] = 8 *

ϑ (°C) + 44 (DS1921Z)

A value of 23°C, for example, thus translates into 68 decimal or 44h for the DS1921H, and 228 decimal

or E4h for the DS1921Z. This corresponds to the binary patterns 0100 0100 and 1110 0100 respectively,

which could be written to a Temperature Alarm Register (address 020Bh and 020Ch, respectively).

Temperature Alarm Register Map

ADDR

020Bh

Temperature Low Alarm Threshold

020Ch

Temperature High Alarm Threshold

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