DS18B20

Arduino library for the DS18B20 sensor - restricted to one sensor per pin.

Arduino Temperature Control Library (ATCL)

This DS18B20 library is not a full featured library for the DS18B20 family. This library supports only one DS18B20 per Arduino/ MCU pin.

If you need more functions or control over the DS18B20 family I refer to the library of Miles Burton - https://github.com/milesburton/Arduino-Temperature-Control-Library

I'm a great fan of the above library however some time ago I needed to strip it down to save a few dozen bytes. I reworked that minimalistic version into a library and I added a number of Arduino examples to help you get started.

Interface

The DS18B20 library supports only the DS18B20, only one sensor per pin, no parasite mode, no Fahrenheit and no alarm functions. The only feature the class supports is the asynchronous reading of the temperature by means of three core functions:

DS18B20(OneWire * ow) constructor needs a reference to OneWire object.
bool begin(uint8_t retries = 3) resets oneWire and set resolution to 9 bit.
returns true if all is OK. there will be a number of retries to connect, default 3.
void requestTemperatures() trigger temperature conversion.
bool isConversionComplete() check if conversion is complete.
float getTempC() returns temperature -127 = DEVICE_DISCONNECTED
void setResolution(uint8_t resolution = 9) resolution = 9..12 (9 is default)
bool getAddress() returns true if the sensor is configured (available)

This allowed the class to be both minimal in size and non-blocking. In fact the class has no support for a synchronous read in one call. This choice will teach people how to work in a non-blocking way from the start.

Effort has been taken to keep the code, variables and function names compatible with ATCL library mentioned above. This way you can step over to that one with relatively few problems when you need more functionality like multiple sensors on one pin.

Finally this library will probably make it easier to use a DS18B20 with processing boards or IC's with small memory footprint.

Operation

This library supports only one DS18B20 per Arduino/ MCU pin.

    //  BOTTOM VIEW
    //
    //          PIN  MEANING
    //   /---+
    //  /  o |  1    GND
    //  |  o |  2    DATA
    //  \  o |  3    VCC
    //   \---+

Connect a pull-up resistor 4.7 KOhm between pin3 and pin2. When the wires are longer this resistor needs to be smaller.

Check examples.

Pull up resistor

An indicative table for pull up resistors, (E12 series), to get started.

Note: thicker wires require smaller resistors (typically 1 step in E12 series)

Length	- 5.0 Volt	- 3.3 Volt
10cm (4")	10K0	6K8
20cm (8")	8K2	4K7
50cm (20")	4K7	3K3
100cm (3'4")	3K3	2K2
200cm (6'8")	2K2	1K0
500cm (16'8")	1K0	*
longer	*	*

* = no info, smaller

Diagnosis notes

It was noted that the library sometimes give unexpected values, and keep sending these values.

This is due to the fact that by default the CRC is not checked to speed up reading. In fact, default only the two temperature registers are read. By setting sensor.setConfig(DS18B20_CRC); the whole scratchpad is read and the CRC can be checked.

table of known "strange values" and actions one could take. It is meant to start some diagnosis.

value	possible cause	optional action
0.0000	data line has no pull up	use pull up
-0.0625	data line is constantly pulled HIGH	check GND
-128	CRC error	wrong pull up, bad sensor ?
-127	DISCONNECTED	check wires }

If a value occurs only once in a while, wiring is often the cause, or it can be caused by e.g. induction e.g. switching on a motor while sensor is read.

Credits

Miles Burton who originally developed the Arduino Temperature Control Library. and all people who contributed to that lib.

Future

unit tests
- add constants
- get it working

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