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Table of Contents

Name

DS18B20 - Programmable Resolution 1-Wire Digital Thermometer

Synopsis

Thermometer.

28 [.]XXXXXXXXXXXX[XX][/[ die | fasttemp | power | temperature | temphigh | templow | trim | trimblanket | trimvalid | address | crc8 | id | locator | present | r_address | r_id | r_locator | type ]]

Family Code

28

Special Properties

fasttemp

read-only, floating point
Low resolution but faster temperature reading by the chip. (~9 bits). Units are selected from the invoking command line. See owfs(1) or owhttpd(1) for choices. Default is Celsius. Conversion takes 94 msec.

die

read-only,ascii
Two character manufacturing die lot. "B6" "B7" or "C2"

power

read-only,yes-no
Is the chip powered externally (=1) or from the parasitically from the data bus (=0)?

temperature

read-only, floating point
Temperature read by the chip at high resolution (~12 bits). Units are selected from the invoking command line. See owfs(1) or owhttpd(1) for choices. Default is Celsius. Conversion takes 750 msec.

temphigh templow

read-write. floating point
Temperature limits for alarms. Units are selected from the invoking command line. See owfs(1) or owhttpd(1) for choices. Default is Celsius.
Note that alarms are not implemented.

trim

read-write,unsigned integer
32 bit trim value in the EEPROM of the chip. When written, it does not seem to read back. Used for a production problem in the B7 die. See AN247.pdf
Read allowed for all chips. Only the B7 chips can be written.

trimblanket

read-write,yes-no
Writing non-zero (=1) puts a default trim value in the chip. Only applied to the B7 die. Reading will be true (non-zero) if trim value is the blanket value. Again, only B7 chips will register true, and since the written trim values cannot be read, this value may have little utility.

trimvalid

read-only,yes-no
Is the trim value in the valid range? Non-zero if true, which includes all non-B7 chips.

Standard Properties

address

r_address

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.

crc8

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).

family

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

id

r_id

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.

locator

r_locator

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

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.

type

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

Alarms

None implemented.

Description

1-Wire

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.

Ds18b20

The DS18B20 (3) is one of several available 1-wire temperature sensors. It is the replacement for the DS18S20 (3) Alternatives are DS1822 (3) as well as temperature/vlotage measurements in the DS2436 (3) and DS2438 (3) . For truly versatile temperature measurements, see the protean DS1921 (3) Thermachron (3) .
Although the DS18B20 (3) can select between 4 resolutions, this program simplifies the interface by only implementing the fastest/roughest and slowest/best.

Addressing

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

Datasheet

http://pdfserv.maxim-ic.com/en/ds/DS18B20.pdf

See Also

Programs

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

Language bindings

owtcl (3) owperl (3)

Clocks

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

ID

DS2401 (3) DS1990A (3)

Memory

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)

Switches

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

Temperature

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

Humidity

DS1922 (3)

Voltage

DS2450 (3)

Resistance

DS2890 (3)

Multifunction

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

Counter

DS2423 (3)

LCD Screen

LCD (3) DS2408 (3)

Crypto

DS1977 (3)

Availability

http://owfs.sourceforge.net

Author

Paul Alfille (palfille@earthlink.net)


Table of Contents

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