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DS18B20 digital thermometer (I) electrical characteristics, parasitic power supply mode and remote wiring

2022-06-12 10:54:00 【Milton】

Catalog

DS18B20 Digital thermometer ( One ) Electrical characteristics , Parasitic power supply mode and remote wiring
DS18B20 Digital thermometer ( Two ) Thermometry , ROM and CRC Algorithm
DS18B20 Digital thermometer ( 3、 ... and ) 1-WIRE Bus ROM search algorithm

DS18B20

DS18B20 Is a common digital thermometer chip , Because the temperature measurement is accurate , Cheap and easy to connect , Widely used in practice , In all kinds of teaching experiment suits, the appearance rate is also very high .

Doing it FwLib_STC8 GPIO Example I wrote it when I was DS18B20, This model looks simple , But using the mechanism is more interesting , I can't write in one space , So take out the content and introduce it separately .

Parameters

1-Wire Bus Bus structure , It is allowed to connect multiple on one bus DS18B20 And communicate separately
At ordinary temperature , You can take power directly from the data port , At this time, only two wires are needed .
Supply voltage [3.0V, 5.5V]
Temperature detection range [-55°C, +125°C] Centigrade , [-67°F, +257°F] Fahrenheit
Accuracy : stay [-10°C, +85°C] by ±0.5°C

Parameter description

The error under normal temperature shall not exceed ±0.5°C
Cheap for domestic products DS18B20 Actually measured , Basically in this range to , At room temperature of twenty or thirty degrees Celsius , The individual error of the actual test is ±0.35°C within .
Low power consumption
Single DS18B20 Use one 0.1uF The capacitor can drive
Temperature monitoring can be carried out through three wire or two-wire connection
Get rid of Vcc and GND, In fact, only one is needed IO Line , Very economical MCU Of IO resources
Support the connection of multiple chips on a single line bus
Only one communication bus is required IO, This line can also be connected to multiple at the same time DS18B20 signal communication , And the distance is very long , It can reach hundreds of meters .

About the maximum communication length and DS18B20 Number of nodes
1-Wire Bus On the bus DS18B20 The quantity and distance of are related to the wiring form , Fifty or sixty are connected at a short distance of more than ten centimeters DS18B20 No problem , If it is a long distance connection of hundreds of meters , Suggest 10 Within a , Don't exceed at most 20 individual . The above premise is that the power supply is sufficient . DS18B20 These characteristics are particularly advantageous in industrial environments , For example, one IDC Computer room , Only one of dozens of temperature measuring points is needed 8 position MCU You can complete the collection , Low power consumption and reliability .

Pin foot

What you usually see is 3pin Of To-92 encapsulation , Like an ordinary triode , Make the plane face towards yourself , Pin Feet down , From left to right is : GND, DQ, VDD

Single DS18B20 The wiring of the cable

Single DS18B20 Is the most basic way to connect

Normal power supply mode

The normal power supply mode uses a three wire connection , The voltage can be selected 3.3V or 5V

MCU IO -> DQ
GND -> GND
5V/3.3V -> VDD

Parasitic power supply mode

The parasitic power supply mode uses a two-wire connection . Now DS18B20 Of GND and Vdd Be grounded . DQ The pin is both a data communication pin , It is also a power supply pin , The upper computer needs to use a pull-up resistor on this pin to connect to VCC On , about STC8, It can be done through registers PxPU Set it up .

It should be noted that , Is not All online purchases DS18B20 Can work in parasitic power supply mode , There are some batch numbers DS18B20 It can not work under the circuit of parasitic power supply mode , Reading only outputs 0. I suspect it's cutting corners , The capacitor and diode needed for parasitic power supply are omitted . For this kind of DS18B20, Some additional circuits are needed to make it work in two-wire mode .

MCU IO -> DQ
GND -> GND & VDD

Simulate parasitic power supply mode

If DS18B20 Cannot operate in parasitic power supply mode , You can use one 0.1uF A capacitor and a 1N4148 Diodes realize two-wire connection . Now DS18B20 In fact, it works in normal power supply mode .

        +-----1N4148-|>|-----+
        |                    |
        |     |DS18B20|-VCC--+
        |     |       |      |
MCU IO--+-DQ--|DS18B20|     0.1uF
              |       |      |
GND   ----GND-|DS18B20|-GND--+

The parasitic power supply mode is simulated by bread board , There are only two wires behind it
DS18B20 Simulate parasitic power supply mode

Multiple DS18B20 connection

In the actual scene , DS18B20 Often use in groups , Used to collect temperature information of an area , The area spans from tens of centimeters ( The case , Cabinet , Lathe ), To hundreds of meters ( residential , Computer room , workshop ) It's possible . There are different choices for different distances and environments , To sum up, there are several situations

You can refer to this one 1-wire Bus wiring https://www.loxone.com/enen/kb/wiring-1-wire-devices/

Bus connection mode

In the following structure S representative Sensor, DS18B20. MCU It's MCU .

Bus mode is the recommended wiring mode , be-all DS18B20 All connected to the same line . The longest distance communication can be achieved by using bus connection


MCU-------8m---S---3m----S----3m----S-----10m-----S

Or there are individual forks , The fork is very close to the bus


MCU-------8m----------+--S---3m----S--+--10m-----S
                      |               |
                      1m              1m
                      |               |
                      S               S

Star connection mode

If more and longer forks occur on the bus , It becomes a star connection , Similar to the following wiring method , Star connection is only recommended for small area scenarios , Compared with the bus connection mode , The length is much shorter .

   S---------8m----MCU
                    |
                   4m
                    |
      S------6m-----+----3m------S
                    |
                   8m
                    |
   S--+------4m-----+----6m------------S
      |
     2m
      |
      S

Power supply mode

Centralized power supply

Most scenarios use centralized power supply . In this scenario , By the control end (MCU End ) Power supply .

If three wires are used, there is no need to pay attention to , If two-wire connection is used , We need to pay attention to

Because communication IO At the same time, it is responsible for power supply , So you need to configure the pull-up resistor , Pull up resistor in short distance scene 5KR enough , But in the long-distance scene, it needs to be reduced , You can try to use 2KR The resistance of .
In case of parasitic power supply mode simulated using normal power supply mode , Voltage usage 5V, Capacitors need to be close DS18B20 Deploy , Every DS18B20 Match one 0.1uF The capacitance of .

Terminal power supply

In a conditional scenario , Every DS18B20 It can be supplied separately , At this time, the control end is connected with DS18B20 Common land , Connection communication IO that will do . For the sake of stability DS18B20 The end can be equipped with 0.1uF capacitance .