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       DS1821 - Programmable Digital Thermostat and Thermometer



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




       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.

       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.

       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.

       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.


       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

       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.

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

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


       read-only, ascii
       Chip type: DS1821


       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.

       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.


       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).



       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)

       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)



       Paul Alfille (