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 | r_address | r_id | r_locator | type ]]





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.




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 preceding 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.

present (DEPRECATED)

read-only, yes-no

Is the device currently present on the 1-wire bus?


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 Semiconductor, Inc. The bus is a low-power low-speed low-connector scheme where the data line can also provide power.

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

The 1-wire scheme uses a single bus master and multiple slaves on the same wire. The bus master initiates all communication. The slaves can be individually discovered and addressed using their unique ID.

Bus masters come in a variety of configurations including serial, parallel, i2c, network or USB adapters.

OWFS design

OWFS is a suite of programs that designed to 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 are represented as simple files that can be read and written.

Details of the individual slave or master design are hidden behind a consistent interface. The goal is to provide an easy set of tools for a software designer to create monitoring or control applications. There are some performance enhancements in the implementation, including data caching, parallel access to bus masters, and aggregation of device communication. Still the fundemental goal has been ease of use, flexibility and correctness rather than speed.

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 them to a 62-bin histogram.


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:

  • 01.123456789ABC

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.




owfs (1) owhttpd (1) owftpd (1) owserver (1) owdir (1) owread (1) owwrite (1) owpresent (1) owtap (1)

Configuration and testing

owfs (5) owtap (1) owmon (1)

Language bindings

owtcl (3) owperl (3) owcapi (3)


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


DS2401 (3) DS2411 (3) DS1990A (3)


DS1982 (3) DS1985 (3) DS1986 (3) DS1991 (3) DS1992 (3) DS1993 (3) DS1995 (3) DS1996 (3) DS2430A (3) DS2431 (3) DS2433 (3) DS2502 (3) DS2506 (3) DS28E04 (3) DS28EC20 (3)


DS2405 (3) DS2406 (3) DS2408 (3) DS2409 (3) DS2413 (3) DS28EA00 (3)


DS1822 (3) DS1825 (3) DS1820 (3) DS18B20 (3) DS18S20 (3) DS1920 (3) DS1921 (3) DS1821 (3) DS28EA00 (3) DS28E04 (3) EDS0064 (3) EDS0065 (3) EDS0066 (3) EDS0067 (3) EDS0068 (3) EDS0071 (3) EDS0072 (3) MAX31826 (3)


DS1922 (3) DS2438 (3) EDS0065 (3) EDS0068 (3)


DS2450 (3)


DS2890 (3)

Multifunction (current, voltage, temperature)

DS2436 (3) DS2437 (3) DS2438 (3) DS2751 (3) DS2755 (3) DS2756 (3) DS2760 (3) DS2770 (3) DS2780 (3) DS2781 (3) DS2788 (3) DS2784 (3)


DS2423 (3)

LCD Screen

LCD (3) DS2408 (3)


DS1977 (3)


DS2406 (3) TAI8570 EDS0066 (3) EDS0068 (3)


EEEF (3) DS2438 (3)



Paul Alfille ([email protected])