Ubuntu Manpage: DS18B20

name
synopsis
family code
special properties
temperature alarm limits
temperature resolution default value
temperature errata
standard properties
description
addressing
datasheet
see also
availability
author

NAME

       DS18B20
              - Programmable Resolution 1-Wire Digital Thermometer

       MAX31820
              - Ambient Temperature Sensor

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 ]]

FAMILY CODE

SPECIAL PROPERTIES

   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
       Measured temperature with 12 bit resolution.

   temperature9 temperature10 temperature11 temperature12
       read-only, floating point
       Measured temperature at 9 to 12 bit resolution. There is a  tradeoff  of  time  versus  accuracy  in  the
       temperature measurement.

   latesttemp
       read-only, floating point
       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.

   fasttemp
       read-only, floating point
       Equivalent to temperature9

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  temperature  scale that was set for temperature
       measurements.

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

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

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

TEMPERATURE RESOLUTION DEFAULT VALUE

   tempres
       read-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.

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

   errata/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.

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

   errata/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.

   errata/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 hexadecimal 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 preceding 56 bits of
the unique ID number. Given as upper case hexadecimal digits (0-9A-F).

family
read-only, ascii
The 8-bit family code. Unique to each type of device. Given as upper case hexadecimal 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 hexadecimal 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 hexadecimal) 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?

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

DESCRIPTION

1-Wire
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 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 fundemental 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.

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 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.

DATASHEET

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

AVAILABILITY

       http://www.owfs.org

AUTHOR

       Paul Alfille (paul.alfille@gmail.com)