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NAME

       DS1821 - Programmable Digital Thermostat and Thermometer

SYNOPSIS

       Thermostat.

       thermostat  [/[  temperature  |  temphigh  |  templow  | temphighflag |
       templowflag | thermostatmode | polarity | 1shot ]]

FAMILY CODE

       none

SPECIAL PROPERTIES

   temperature
       read-only, floating point
       9-bit resolution temperature. Units  are  selected  from  the  invoking
       command  line.  See  owfs(1)  or  owhttpd(1)  for  choices.  Default is
       Celsius.  Conversion takes 1 second.

   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.

   temphighflag, templowflag
       read-write. yes-no
       temphighflag goes to 1 when temperature exceeds temphigh and stays at 1
       until  it  is  reset by writing it to zero.  This state persists across
       power  cycles  and  operates  in  both  1-wire  and  thermostat  modes.
       templowflag  behaves  in  the same way, monitoring the temperature with
       respect to templow.

   thermostatmode
       read-write. yes-no
       When this bit is set to 1 the chip will enter thermostat  mode  on  the
       next power-up.  See the datasheet for further information on thermostat
       mode.  NOTE: Once the DS1821 has entered thermostat mode it  cannot  be
       taken  out  back  to  1-wire  mode  using  a 1-wire interface.  Special
       electrical incantations on the power and data lines must  be  performed
       that  are  not  possible  with  a  normal  1-wire bus master. A special
       circuit is required - see the datasheet for complete details.

   polarity
       read-write. yes-no
       Controls the output sense  of  the  thermostat  output  (DQ)  while  in
       thermostat  mode.   If polarity is 0 the output is active low. If it is
       1, active high.

   1shot
       read-write. yes-no
       If this bit is 1, a START CONVERT command will begin a  conversion  and
       the  chip will enter a low power state when the conversion is complete.
       If the bit is 0 then START CONVERT will begin a  conversion  and  start
       another  one  as soon as it is done.  STOP CONVERSION must be performed
       to get the conversion cycle  to  stop.   This  interface  automatically
       issues a STOP CONVERSION command when going out of continuous mode.

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.

STANDARD PROPERTIES

   type
       read-only, ascii
       Chip type: DS1821

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 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.

   DS1821
       The  DS1821  (3)  is  a  unique 1-wire device. It is unaddressable, and
       therefore there can be only one on a given bus.   It  is  meant  to  be
       programmed  once  using  1-wire  and  then  permanently  installed in a
       thermostat  circuit.   Once  in  thermostat  mode,  it  is  no   longer
       programmable  from  a  1-wire  interface.  It is possible to recover it
       into 1-wire mode but only with a special circuit.

ADDRESSING

       Unlike all other 1-wire chips, the ,B DS1821 (3) has no unique address.
       It  is  addressed  as thermostat and will not announce itself in device
       discovery (directory listing).

DATASHEET

       http://pdfserv.maxim-ic.com/en/ds/DS1821.pdf

SEE ALSO

   Programs
       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)

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

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

   Memory
       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)

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

   Temperature
       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)

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

   Voltage
       DS2450 (3)

   Resistance
       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)

   Counter
       DS2423 (3)

   LCD Screen
       LCD (3) DS2408 (3)

   Crypto
       DS1977 (3)

   Pressure
       DS2406 (3) -- TAI8570 EDS0066 (3) EDS0068 (3)

AVAILABILITY

       http://www.owfs.org

AUTHOR

       Paul Alfille (paul.alfille@gmail.com)