'\"
'\" Copyright (c) 2003-2004 Paul H Alfille, MD
'\" (paul.alfille@gmail.com)
'\"
'\" Device manual page for the OWFS -- 1-wire filesystem package
'\" Based on Dallas Semiconductor, Inc's datasheets, and trial and error.
'\"
'\" Free for all use. No warranty. None. Use at your own risk.
'\"
.TH DS1821 3  2003 "OWFS Manpage" "One-Wire File System"
.SH NAME
.B DS1821
\- Programmable Digital Thermostat and Thermometer
.SH SYNOPSIS
Thermostat.
.PP
.B thermostat
[/[
.B temperature
|
.B temphigh
|
.B templow
|
.B temphighflag
|
.B templowflag
|
.B thermostatmode
|
.B polarity
|
.B 1shot
]]
.SH FAMILY CODE
.PP
.I none
.SH SPECIAL PROPERTIES
.SS temperature
.I read-only, floating point
.br
9-bit resolution temperature. Units are selected from the invoking command line. See
.B owfs(1)
or
.B owhttpd(1)
for choices. Default is Celsius.
Conversion takes 1 second.
.SS temphigh templow
.I read-write. floating point
.br
Temperature limits for alarms. Units are selected from the invoking command line. See
.B owfs(1)
or
.B owhttpd(1)
for choices. Default is Celsius.
.br
Note that alarms are not implemented.
.SS temphighflag, templowflag
.I read-write.  yes-no
.br
.B temphighflag
goes to 1 when
.B temperature
exceeds
.B 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.
.B templowflag 
behaves in the same way, monitoring the temperature with respect to 
.B templow.
.SS thermostatmode
.I read-write. yes-no
.br
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.
.B 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.
.SS polarity
.I read-write. yes-no
.br
Controls the output sense of the thermostat output (DQ) while in thermostat mode.
If 
.B polarity
is 0 the output is active low. If it is 1, active high.
.SS 1shot
.I read-write. yes-no
.br
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.
.so man3/temperature_threshold.3so
.SH STANDARD PROPERTIES
.SS type
.I read-only, ascii
.br
Chip type: DS1821
.SH DESCRIPTION
.so man3/description.3so
.SS DS1821
The
.B 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.

.SH ADDRESSING
Unlike all other 1-wire chips, the 
,B DS1821 (3)
has no unique address. It is addressed as
.B thermostat
and will not announce itself in device discovery (directory listing).
.SH DATASHEET
.br
http://pdfserv.maxim-ic.com/en/ds/DS1821.pdf
.SH SEE ALSO
.so man3/seealso.3so
.SH AVAILABILITY
http://www.owfs.org
.SH AUTHOR
Paul Alfille (paul.alfille@gmail.com)