NAME

DS1821 - Programmable Digital Thermostat and Thermometer

SYNOPSIS

Thermostat.

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.

TEMPERATURE ALARM LIMITS

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.

temphigh

read-write, integer
Shows or sets the lower limit for the high temperature alarm state.

templow

read-write, integer
Shows or sets the upper limit for the low temperature alarm state.

STANDARD PROPERTIES

type

read-only, ascii
Chip type: DS1821

DESCRIPTION

1-Wire

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 design

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.

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

AVAILABILITY

http://www.owfs.org

AUTHOR

Paul Alfille (paul.alfille@gmail.com)