DS18B20

Addressing

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

       FamilyCode
              8 bits

       Address
              48 bits

       CRC    8 bits

       Addressing under OWFS is in hexadecimal, 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.

Author

       Paul Alfille (paul.alfille@gmail.com)

OWFS Manpage                                          2003                                            DS18B20(3)

Availability

http://www.owfs.org

Datasheet

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

Description

1-Wire1-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 busmaster 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.

OWFSdesignOWFS is a suite of programs that designed to make the 1-wire bus and its devices easily accessible. The
underlying principle 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 fundamental goal has been ease of use,
flexibility and correctness rather than speed.

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/voltage measurements in the DS2436(3) and
DS2438(3). For truly versatile temperature measurements, see the protean DS1921(3)Thermachron(3).

The MAX31820 is functionally identical to the DS18B20 except for it's voltage requirements.
The DS18B20(3) can select between 4 resolutions. In general, high resolution is the best choice unless
your application is truly time-constrained.

Family Code

Name

DS18B20
              - Programmable Resolution 1-Wire Digital Thermometer

       MAX31820
              - Ambient Temperature Sensor

Special Properties

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

   temperatureread-only,floatingpoint
       Measured temperature with 12 bit resolution.

   temperature9temperature10temperature11temperature12read-only,floatingpoint
       Measured temperature at 9 to 12 bit resolution. There is a  tradeoff  of  time  versus  accuracy  in  the
       temperature measurement.

   latesttempread-only,floatingpoint
       Measured  temperature  at 9 to 12 bit resolution, depending on the resolution of the latest conversion on
       this chip. Reading this node will never trigger a temperature conversion. Intended for use in conjunction
       with /simultaneous/temperature.fasttempread-only,floatingpoint
       Equivalent to temperature9

Standard Properties

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

crc8read-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 hexadecimal digits (0-9A-F).

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

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

locatorr_locatorread-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 LinkLocator the locatorwillshowaunique8-bytenumber(16characterhexadecimal)startingwithfamilycodeFE.
If no LinkLocator is between the device and the master, the locator field will be all FF.
rlocator is the locator in reverse order.

present(DEPRECATED)read-only,yes-no
Is the device currently present on the 1-wire bus?

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

Synopsis

       Thermometer.

       28  [.]XXXXXXXXXXXX[XX][/[  fasttemp  |  temperature  |  temperature9  |  temperature10 | temperature11 |
       temperature12 | latesttemp | die | power | temphigh | templow | tempres  |  errata/die  |  errata/trim  |
       errata/trimblanket  |  errata/trimvalid  | address | crc8 | id | locator | r_address | r_id | r_locator |
       type ]]

Temperature Alarm Limits

       When the device exceeds either temphigh or templow temperature threshold  the  device  is  in  the  alarm
       state,  and  will  appear in the alarm directory. This provides an easy way to poll for temperatures that
       are unsafe, especially if simultaneous temperature conversion is done.

       Units for the temperature alarms are  in  the  same  temperaturescale  that  was  set  for  temperature
       measurements.

       Temperature  thresholds  are  stored  in  non-volatile memory and persist until changed, even if power is
       lost.

   temphighread-write,integer
       Shows or sets the lower limit for the high temperature alarm state.

   templowread-write,integer
       Shows or sets the upper limit for the low temperature alarm state.

Temperature Errata

       There are a group of obscure internal properties exposed to protect against an hardware defect in certain
       batches of the B7 die  of  some  DS18x20  chips.  See  http://www.1wire.org/en-us/pg_18.html  or  request
       AN247.pdf from Dallas directly.

   errata/dieread-only,ascii
       Two character manufacturing die lot. "B6" "B7" or "C2"

   errata/trimread-write,unsignedinteger
       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.

       Read allowed for all chips. Only the B7 chips can be written.

   errata/trimblanketread-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.

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

Temperature Resolution Default Value

tempresread-write,integer
       The device employs a non-volatile memory to store the default temperature resolution (9,  10,  11  or  12
       bits)  to  be  applied  after  power-up.  This is useful if you use simultaneous temperature conversions.
       Reading this node gives you the value stored in the non-volatile memory.  Writing  sets  a  new  power-on
       resolution value.

       As  a  side  effect, reading this node resets the temperature resolution used by simultaneous temperature
       conversions to its power-on value. It also affects the resolution value used by latesttemp, to scale  the
       latest  conversion  value,  so  make  sure to re-sample the temperature before accessing latesttemp after
       writing or reading the tempres value.