DS1821 − Programmable Digital Thermostat and Thermometer
Thermostat.
thermostat [/[ temperature | temphigh | templow | temphighflag | templowflag | thermostatmode | polarity | 1shot ]]
none
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.
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.
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.
read-only,
ascii
Chip type: 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.
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).
http://pdfserv.maxim-ic.com/en/ds/DS1821.pdf
http://www.owfs.org
Paul Alfille ([email protected])