| DS1821(3) | One-Wire File System | DS1821(3) |
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.
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.
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 is a suite of programs that designed to make the 1-wire bus and its devices easily accessible. The underlying principle 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 fundamental 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).
http://pdfserv.maxim-ic.com/en/ds/DS1821.pdf
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owfs (5) owtap (1) owmon (1)
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http://www.owfs.org
Paul Alfille (paul.alfille@gmail.com)
| 2003 | OWFS Manpage |