NAME

DS1825 - Programmable Resolution 1-Wire Digital Thermometer with ID

MAX31826: - Digital Temperature Sensor with 1Kb Lockable EEPROM
MAX31850 MAX31851: - Cold-Junction Compensated Thermocouple

SYNOPSIS

Thermometer with hardware address pins (DS1825)

Digital Temperature Sensor with 1Kb Lockable EEPROM (MAX31826)

Cold-Junction Compensated Thermocouple (MAX31850 and MAX31851)

FAMILY CODE

SPECIAL PROPERTIES

power

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

prog_addr

read-only, 0-15
A distiguishing feature of the DS1825 (3) is the ability to set hardware pins for an address (0-15). This is an alternative to the unique 64-bit address that is set in the factory.

temperature

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

For the MAX31850 MAX 31851 this is the cold-junction temperature -- the temperature at the chip. See thermocouple

temperature9 temperature10 temperature11 temperature12

read-only, floating point ( DS1825 only)

Measured temperature at 9 to 12 bit resolution. There is a tradeoff of time versus accuracy in the temperature measurement.

The MAX31826 MAX31850 MAX31851 measure all temperatures at 12 bit resoltion and will return that resolution to all the possible temperature properties.

latesttemp

read-only, floating point
Measured temperature at 9 to 12 bit resolution, depending on the resolution of the latest conversion on this chip. Reading this node will never trigger a temperature conversion. Intended for use in conjunction with /simultaneous/temperature.

fasttemp

read-only, floating point
Equivalent to temperature9

thermocouple

read-only, floating point ( MAX31850 MAX31851 only)

Measured temperature of the thermocouple at 16bit resolution. Cold-junction temperature compensated.

The actual thermocouple type used is set by the selected chip type, and is not discoverable in software.

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.

The MAX31826 does NOT have temperature thresholds and temperature alarm.

TEMPERATURE RESOLUTION DEFAULT VALUE

tempres

read-write, integer
The device employs a non-volatile memory to store the default temperature resolution (9, 10, 11 or 12 bits) to be applied after power-up. This is useful if you use simultaneous temperature conversions. Reading this node gives you the value stored in the non-volatile memory. Writing sets a new power-on resolution value.

As a side effect, reading this node resets the temperature resolution used by simultaneous temperature conversions to its power-on value. It also affects the resolution value used by latesttemp, to scale the latest conversion value, so make sure to re-sample the temperature before accessing latesttemp after writing or reading the tempres value.

MEMORY

Only the MAX31826 supports memory functions.

pages/page.0 .. pages/page.15 pages/page.ALL

read/write, binary
EEPROM memory pages of 8 bytes each. See the datasheet about locking contents.

memory

read/write, binary
EEPROM memory of 128 bytes. See the datasheet about locking contents.

FAULT REPORTING

Only the MAX31850 MAX31851 supports fault reporting.

fault

read-only, yes-no
Fault in last thermocouple conversion

open_circuit

read-only, yes-no
Thermocouple leads disconnected.

ground_short

read-only, yes-no
Thermocouple lead shorted to ground.

vdd_short

read-only, yes-no
Thermocouple lead shorted to supply voltage.

STANDARD PROPERTIES

address

r_address

read-only, ascii
The entire 64-bit unique ID. Given as upper case hexadecimal 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 hexadecimal digits (0-9A-F).

family

read-only, ascii
The 8-bit family code. Unique to each type of device. Given as upper case hexadecimal 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 hexadecimal 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 hexadecimal) 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 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.

DS1825

The DS1825 (3) is one of several available 1-wire temperature sensors. Alternatives are the DS18S20 (3), DS18B20 (3), and 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 DS1825 (3) can select between 4 resolutionsspanning the fastest/roughest and slowest/best.

MAX31826

The MAX31826 shares a family code with the DS1825 but has differences in some of its functions.

The MAX31826 has 128 btes of EEPROM memory (as 16 pages of 8 bytes) while the DS1825 has no memory available.

The MAX31826 measures temperature at 12 bit resolution as fast as the DS1825's lowest resolution (and always uses 12-bit resolution). On the other hand it has no temperature thresholds or alarm function.

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 hexadecimal, 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.

Both the MAX31826 and the DS1825 allow hardware selection of part of the address, which can assist selecting between chips is some circuit designs.

DATASHEET

DS1825: http://pdfserv.maxim-ic.com/en/ds/DS1825.pdf
MAX31826: http://datasheets.maxim-ic.com/en/ds/MAX31826.pdf

AVAILABILITY

http://www.owfs.org

AUTHOR

Paul Alfille (paul.alfille@gmail.com)