ONE WIRE PROTOCOLTEMPERATURE SENSORS:LM35 vs. DS18B20

Contents1.Introduction

1-wire protocol

2.How do they work ?

4.Code examples

3.Temperature sensor : LM35 vs. DS18B20

1. Overview of OWPOWP : copyright by MAXIMFeatures:

Bi-directionalSerial signal, half-duplexPower auto charge(by the internal capacitor in

slaves)One Master controls many slaves (different IDs)

in single line

1. Overview of OWP

1. Overview of OWP

2.WorkingCommunication with time slotsSlave self-clocked from master edge (high to

low pulse)Time slot:

+bit 1: pull bus low in short Tx > 1-2us

+bit 0: pull bus low in long Tx=60-120usSlaves sample in 15-60usMaster sample at 15us

2.Working

ex: The transaction sequence for accessing the DS18B20 is as follows:

Step 1.Reset bus

Step 2. ROM Command

Step 3. Function Command

3.TEMPERATURE SENSORS DS18B20 vs. LM35

3.1 DS18B20 -DIGITAL SIGNAL

Unique 1-Wire® Interface Requires Only One Multi-drop Capability Simplifies Distributed Temperature-Sensing Applications Requires No External Components Can Be Powered from Data Line; Power Supply Range is 3.0V to 5.5V Measures Temperatures from -55°C to +125°C (-67°F

to +257°F) ±0.5°C Accuracy from -10°C to +85°C Thermometer Resolution is User Selectable from 9 to

12 Bits Converts Temperature to 12-Bit Digital Word in 750ms

(Max) (Reference :DS18B20 datasheet)

3.1 DS18B20:RESET

//*************************************************************//ham onewire_reset() reset va xac nhan co ngoai vi tren mach //tra ve bit presence_pulse :0->co device ket noi va nguoc lai//-------------------------------------------------------------void onewire_reset()

onewire_pin_tris=0; //set_output_pinonewire_pin=0;__delay_us(250);__delay_us(230);onewire_pin_tris=1; //set_input_pin realease bus__delay_us(70); //sample presence pulsepresence_pulse=onewire_pin; //return 1:no device;0:device(s)__delay_us(410);

3.1 DS18B20:READ BIT

//master keo xuong low bus >1us roi realease bus lay mau trong khoang 15us//delay them khoang 60us la lay mau xong 1 bit onewire_read_bit()

onewire_pin_tris=0;onewire_pin=0;__delay_us(6);onewire_pin_tris=1; //realease bus __delay_us(8);DQ=onewire_pin; //sample in less than 15us after pull low the line

//return value to bit DQ

3.1 DS18B20:WRITE BIT

//ghi bit 1: ghi 0 trong >1us(chon 6us) sau do release bus trong >60us//ghi bit 0: ghi 0 trong 60-120us( chon 65us) sau do realese bus >1us(chon 5us )void onewire_write_bit1()

onewire_pin_tris=0; //pull low the lineonewire_pin=0;__delay_us(6);onewire_pin_tris=1; //realease bus__delay_us(64);

void onewire_write_bit0()

onewire_pin_tris=0;onewire_pin=0;__delay_us(65); //at least 60usonewire_pin_tris=1; //realease bus;recovery time__delay_us(5); //recovery time

DS18B20 Memory Map

3.1 DS18B20: LCD DISPLAY TEMP

Step1

•Initialization MCU , LCD…

•Reset and detect any 1-wire device connected on the bus

•If (no device) reset again

Step2

•Write Rom command :Skip Rom (only one slave on the bus)

•Write Function Command: Convert T

•Wait for 750ms

•Write Function Command: Read Scratchpad

Step3

•Convert 2 first data bytes in Scratchpad to ASCII

• Display temperature on LCD

•Return step1

3.2 LM35-ANALOG SIGNALCalibrated directly in ˚ Celsius (Centigrade) Linear + 10.0 mV/˚C scale factor0.5˚C accuracy guarantee able (at +25˚C)Low cost due to wafer-level trimmingVs from 4 to 30 voltsLow self-heating, 0.08˚C in still airNonlinearity only ±1⁄4˚C typicalLow impedance output, 0.1 Ω for 1 mA load

(Reference :DS18B20 datasheet)

3.2 LM35: LCD DISPLAY TEMP

Step1

•Initialization MCU , LCD…

•Adc_init();

Step2

•Read_adc();

Step3

•Convert adc to temperature(Vadc=t(C )* 10(mvolt))

• Display temperature on LCD

•Return step1

Simulation Result:

Temperature Sensors

DS18B20 LM35

Signal transmit -digital signal -analog signal

Data Process -1-wire protocol(1 master + many slaves)

-analog digital conversion

Resolution -0.0625 C -10mv/C + ADC 10bit0.5C

Price(Tme_30/5/2011) -27000VND -22000VND

Source code: (tested on board)Onewire.c ; onewire.hMPLAB

project:PIC_DS18B20 ;PIC_temp_sensorProteus file

References:

-All about OWP-1-Wire Overview-DS18B20 datasheet-LM35 datasheet-PIC16F887 datasheet