DS1821(3)

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.

STANDARD PROPERTIES

  type
read-only, ascii
Chip type: DS1821

DESCRIPTION

  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

AVAILABILITY
http://www.owfs.org

AUTHOR
Paul Alfille (paul.alfille@gmail.com)