Table of Contents
- Programmable Digital Thermostat and Thermometer
thermostat [/[ temperature | temphigh | templow | temphighflag | templowflag
| thermostatmode | polarity | 1shot ]]
9-bit resolution temperature. Units are selected from the invoking command
line. See owfs(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)
for choices. Default is Celsius.
Note that alarms are not implemented.
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.
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.
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.
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.
thresholds are stored in non-volatile memory and persist until changed,
even if power is lost.
Shows or sets the lower limit for the high temperature alarm state.
Shows or sets the upper limit for the low temperature alarm state.
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
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.
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
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.
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).
TAI8570 EDS0066 (3)
Paul Alfille (email@example.com)
Table of Contents