DS18B20(3) manual page

Name

DS18B20: Programmable Resolution 1-Wire Digital Thermometer
MAX31820: Ambient Temperature Sensor

Family Code

Special Properties

power

read-only,yes-no
Is the chip powered externally (=1) or from the parasitically from the data bus (=0)?

temperature

read-only, floating point
Measured temperature with 12 bit resolution.

temperature9 temperature10 temperature11 temperature12

read-only, floating point Measured temperature at 9 to 12 bit resolution. There is a tradeoff of time versus accuracy in the temperature measurement.

fasttemp

read-only, floating point
Equivalent to temperature9

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.

Temperature Errata

There are a group of obscure internal properties exposed to protect against an hardware defect in certain batches of the B7 die of some DS18x20 chips. See http://www.1wire.org/en-us/pg_18.html or request AN247.pdf from Dallas directly.

errata/die

read-only,ascii
Two character manufacturing die lot. "B6" "B7" or "C2"

errata/trim

read-write,unsigned integer
32 bit trim value in the EEPROM of the chip. When written, it does not seem to read back. Used for a production problem in the B7 die.

Read allowed for all chips. Only the B7 chips can be written.

errata/trimblanket

read-write,yes-no
Writing non-zero (=1) puts a default trim value in the chip. Only applied to the B7 die. Reading will be true (non-zero) if trim value is the blanket value. Again, only B7 chips will register true, and since the written trim values cannot be read, this value may have little utility.

errata/trimvalid

read-only,yes-no
Is the trim value in the valid range? Non-zero if true, which includes all non-B7 chips.

Standard Properties

address

r_address

read-only, ascii
The entire 64-bit unique ID. Given as upper case hexidecimal digits (0-9A-F).
address starts with the family code
r address is the address in reverse order, which is often used in other applications and labeling.

crc8

read-only, ascii
The 8-bit error correction portion. Uses cyclic redundancy check. Computed from the preceding 56 bits of the unique ID number. Given as upper case hexidecimal digits (0-9A-F).

family

read-only, ascii
The 8-bit family code. Unique to each type of device. Given as upper case hexidecimal digits (0-9A-F).

id

r_id

read-only, ascii
The 48-bit middle portion of the unique ID number. Does not include the family code or CRC. Given as upper case hexidecimal digits (0-9A-F).
r id is the id in reverse order, which is often used in other applications and labeling.

locator

r_locator

read-only, ascii
Uses an extension of the 1-wire design from iButtonLink company that associated 1-wire physical connections with a unique 1-wire code. If the connection is behind a Link Locator the locator will show a unique 8-byte number (16 character hexidecimal) starting with family code FE.
If no Link Locator is between the device and the master, the locator field will be all FF.
r locator is the locator in reverse order.

present (DEPRECATED)

read-only, yes-no
Is the device currently present on the 1-wire bus?

type

read-only, ascii
Part name assigned by Dallas Semi. E.g. DS2401 Alternative packaging (iButton vs chip) will not be distiguished.

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 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.

Ds18b20

The DS18B20 (3) is one of several available 1-wire temperature sensors. It is the replacement for the DS18S20 (3) Alternatives are DS1822 (3) as well as temperature/voltage measurements in the DS2436 (3) and DS2438 (3) . For truly versatile temperature measurements, see the protean DS1921 (3) Thermachron (3) .

The MAX31820 is functionally identical to the DS18B20 except for it’s voltage requirements.
The DS18B20 (3) can select between 4 resolutions. In general, high resolution is the best choice unless your application is truly time-constrained.

Addressing

All 1-wire devices are factory assigned a unique 64-bit address. This address is of the form:

Family Code: 8 bits
Address: 48 bits
CRC: 8 bits

Addressing under OWFS is in hexidecimal, of form:
01.123456789ABC

where 01 is an example 8-bit family code, and 12345678ABC is an example 48 bit address.

The dot is optional, and the CRC code can included. If included, it must be correct.

Temperature Alarm Limits

Temperature Errata

Standard Properties

Description