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DS1921 - Thermachron temperature logging iButton. Thermachron - Thermachron temperature logging iButton.


Temperature logging iButton.

about/[measuring| resolution| samples| templow| temphigh| version] |
clock/[date| running| udate] |
histotgram/[counts[0-62|ALL]| gap| temperature[counts[0-62|ALL]] |
log[date[0-2047|ALL]| elements| temperature[0-2047|ALL]| udate[0-2047|ALL]] |
memory |
mission/[date| delay| easystart| frequency| rollover| running| samples| sampling| udate] |
overtemp/[date[0-11|ALL]| elements| end[0-11|ALL]| count[0-11|ALL]| temperature[0-11|ALL]| udate[0-11|ALL]] |
pages/page.[0-15|ALL] |
temperature |
undertemp/[date[0-11|ALL]| elements|end[0-11|ALL]| count[0-11|ALL]| temperature[0-11|ALL]| udate[0-11|ALL]] | address | crc8 | id | locator | present | r_address | r_id | r_locator | type ]]

Family Code


Special Properties


read-only, yes-no
Is this DS1921 currently measuring a temperature?


read-only, floating point
What is the resolution of the temperature measurments (in the current temperature scale).


read-only, unsigned integer
How many total temperature measurements has this DS1921 performed?


read-only, floating point
Highest temperature this DS1921 can measure (in the current temperature scale).


read-only, floating point
Lowest temperature this DS1921 can measure (in the current temperature scale).


read-only, ascii
Specific version of this DS1921.


read-write, ascii
26 character date representation of the internal time stored in this DS1921. Increments once per second while clock/running
Setting date to a null string will put the current system time.
Accepted date formats are:
Sat[urday] March 12 12:23:59 2001

Apr[il] 4 9:34:56 2002

3/23/04 23:34:57

current locale setting (your system’s format)


read-write, yes-no
Whether the internal clock is running. This can be explicitly set, and is automatically started by setting clock/date or clock/udate or by starting a mission with mission/easystart or mission/frequency

The main reason to stop the clock is to conserve the internal battery. The clock cannot be stopped during a mission, and the clock is essential for a mission.


read-write, unsigned integer
A numeric representation of clock/date
The number of seconds in UNIX time (since Jan 1, 1970).

histogram/counts.0 ... histogram/counts.62 histogram/counts.ALL

read-only, unsigned integer
The number of samples in the current mission whose temperature fell within the histogram/temperature to histogram/temperature+histogram/gap range.


read-only, unsigned integer
The number of bins in the histogram. Always 63.


read-only, floating point
The size of the histogram bin. Depends on the Thermochron version ( about/version ) and is usually 4 times about/resolution

Given in the current temperatature scale.

histogram/temperature.0 ... histogram/temperature.62 histogram/temperature.ALL

read-only, floating point
Lower limit of the temperature range for the corresponding histogram bin. In the current temperature scale.

log/date.0 ... log/date.2047 log/date.ALL

read-only, ascii
Date that the corresponding log/temperature was taken, in ascii format. (See clock/date for more on the format). The number of valid entries is actually log/elements since the log may not be full.

mission/samples gives the total number of samples that have been taken but there is only room in the log for 2048 entries. Once the log is full, mission/rollover determines the Thermochron’s behavior.

If mission/rollover is false(0), the log will hold the first 2048 samples and log/date.0 will always be the same as mission/date

If mission/rollover is true (1) then the log will hold the last 2048 samples and the entries will be shifted down with each new sample.

Note the OWFS code "untwists" the rollover behavior. The data will always be a linear array of earliest to latest.

ALL is the all data elements comma separated.


read-only, unsigned integer
Number of valid entries in the log. OWFS offers the full 2048 values in the log memory, but not that many samples may yet have been taken. log/elements will range from 0 to 2048 and always be less than or equal to mission/samples

log/temperature.0 ... log/temperature.2047 log/temperature.ALL

read-write, floating point
The temperature readings (in the current temperature scale) that correspond to the log/date sample. See log/date for details on the indexing scheme and rollover behavior.

log/udate.0 ... log/udate.2047 log/udate.ALL

read-write, unsigned integer
A numeric representation of log/date
The number of seconds in UNIX time (since Jan 1, 1970).


read-write, binary
User available storage space. 512 bytes. Can also be accessed as 16 pages of 32 bytes with the pages/page.x properties.

overtemp/count.0 ... overtemp/count.11 overtemp/count.ALL

undertemp/count.0 ... undertemp/count.11 undertemp/count.ALL

read-only, unsigned integer
Number of sampling periods that the Thermochron stayed out of range durring a mission. Each sampling period is mission/frequency minutes long.

overtemp/end.0 ... overtemp/end.11 overtemp/end.ALL

undertemp/end.0 ... undertemp/end.11 undertemp/end.ALL

read-only, ascii
End of time that the Thermochron went out of range during the current mission. See clock/date for format.

Each period can be up to 255 samples in length, and span the time overtemp/date to overtemp/end ( or undertemp/date to undertemp/end ).

overtemp/date.0 ... overtemp/date.11 overtemp/date.ALL

undertemp/date.0 ... undertemp/date.11 undertemp/date.ALL

read-only, ascii
Time that the Thermochron went out of range during the current mission. See clock/date for format.



read-only, unsigned integer
Number of entries (0 to 12) in the overtemp or undertemp array.



read-write, floating point
Temperature limit to trigger alarm and error log. overtemp/temperature gives upper limit and undertemp/temperature gives lower limit.
In current temperature scale.

overtemp/udate.0 ... overtemp/udate.11 overtemp/udate.ALL

undertemp/udate.0 ... undertemp/udate.11 undertemp/udate.ALL

read-only, unsigned integer
A numeric representation of overtemp/date or undertemp/date
The number of seconds in UNIX time (since Jan 1, 1970).

pages/page.0 ... pages/page.15 pages/page.ALL

read-write, binary
Memory is split into 16 pages of 32 bytes each. User available. The log memory, register banks and histogram data area are all separate from this memory area.
ALL is an aggregate of the pages. Each page is accessed sequentially.


read-only, floating point
Last temperature explicitly requested. Only available when the mission is not in progress. Value returned in in the current temperature scale.

Standard Properties



read-only, ascii
The entire 64-bit unique ID. Given as upper case hexidecimal digits (0-9A-F).
address starts with the family code
r address is the address in reverse order, which is often used in other applications and labeling.


read-only, ascii
The 8-bit error correction portion. Uses cyclic redundancy check. Computed from the preceeding 56 bits of the unique ID number. Given as upper case hexidecimal digits (0-9A-F).


read-only, ascii
The 8-bit family code. Unique to each type of device. Given as upper case hexidecimal digits (0-9A-F).



read-only, ascii
The 48-bit middle portion of the unique ID number. Does not include the family code or CRC. Given as upper case hexidecimal digits (0-9A-F).
r id is the id in reverse order, which is often used in other applications and labeling.



read-only, ascii
Uses an extension of the 1-wire design from iButtonLink company that associated 1-wire physical connections with a unique 1-wire code. If the connection is behind a Link Locator the locator will show a unique 8-byte number (16 character hexidecimal) starting with family code FE.
If no Link Locator is between the device and the master, the locator field will be all FF.
r locator is the locator in reverse order.


read-only, yes-no
Is the device currently present on the 1-wire bus?
Note, this means that each hex byte (2 characters) is reversed, not the text string.


read-only, ascii
Part name assigned by Dallas Semi. E.g. DS2401 Alternative packaging (iButton vs chip) will not be distiguished.





1-wire is a wiring protocol and series of devices designed and manufactured by Dallas Semicondictor, Inc. The bus is a low-power low-speed low-connector scheme where the data line can also provide power.

Each device is uniquely unalterably numbered during manufacture. There is a wide variety of devices, including memory, sensors (humidity, temperature, voltage, contact, current), switches, timers, loggers, and more complex devices (like thermocouple sensors) can be built wit hte basic devices. There are also 1-wire devices that have encryption included.

The 1-wire bus is accessed via one of a number of serial, parallel or USB adapters.

OWFS design

OWFS is a suite of programs that fundementally make the 1-wire bus and its devices easily accessible. The underlying priciple is to create a virtual filesystem, with the unique ID being the directory, and the individual properties of the device files.

There is optional data caching to improved performance, but possible confusion over stale data.

DS1921 Thermochron

The DS1921 (3) is an iButton device with many intriguing functions. Essentially it monitors temperature, giving both a log of readings, and a histogram of temperature ranges. The specification is somewhat complex, but OWFS hides many of the implementation details.

While on a mission the DS1921 (3) records temperature readings in a 2048-sample log and adds the to 62-bin histogram. more complete form of the DS2438 (3) battery chip. Current sensing is availble, but not implemented. The major advantage compared to the DS2436 (3) is that two voltages can be read, allowing correcting circuit nmeasurements to supply voltage and temperature.


All 1-wire devices are factory assigned a unique 64-bit address. This address is of the form:
Family Code
8 bits
48 bits
8 bits

Addressing under OWFS is in hexidecimal, of form:

where 01 is an example 8-bit family code, and 12345678ABC is an example 48 bit address.

The dot is optional, and the CRC code can included. If included, it must be correct.


http://pdfserv.maxim-ic.com/en/ds/DS2438.pdf http://pdfserv.maxim-ic.com/en/an/humsensor.pdf

See Also


owfs (1) owhttpd (1) owftpd (1) owserver (1)

Language bindings

owtcl (3) owperl (3)


DS1427 (3) DS1904(3) DS1994 (3) DS2404 (3) DS2404S (3) DS2415 (3) DS2417 (3)


DS2401 (3) DS1990A (3)


DS1982 (3) DS1985 (3) DS1986 (3) DS1991 (3) DS1992 (3) DS1993 (3) DS1995 (3) DS1996 (3) DS2431 (3) DS2433 (3) DS2502 (3) DS2506 (3) DS2804 (3)


DS2405 (3) DS2406 (3) DS2408 (3) DS2409 (3) DS2413 (3)


DS1822 (3) DS18B20 (3) DS18S20 (3) DS1920 (3) DS1921 (3) DS1821 (3)


DS1922 (3)


DS2450 (3)


DS2890 (3)


DS2436 (3) DS2438 (3) DS2760 (3)


DS2423 (3)

LCD Screen

LCD (3) DS2408 (3)


DS1977 (3)




Paul Alfille (palfille@earthlink.net)

Table of Contents

© 2003-4 Paul H. Alfille and The owfs Team
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