PIC16F84A DS18B20 1-Wire Temperature Reading And Multiplexing Display Example Using XC8

Device Overview

The DS18B20 is a digital thermometer that able to convert the temperature from -55 to +125 degree Celsius. The controller communicates with this temperature sensor using a 1-Wire bus. So this device requires only three wires including GND, VCC, and data line. Furthermore it can use data line to power  the device eliminating the VCC. 

DS18B20 Pin Configuration

DS18B20 In TO-92 Package

 A TO-92 package is a typical package for hardware experiment.

A Typical Connection Between Microprocessor And DS18B20

 

Using Multiple Devices Over A 1-Wire Bus

It has its own 64-bit ROM unique ID allowing the controller interface to multiple DS18B20 devices over a 1-Wire bus.  

DS18B20 64-Bit ROM Area

 

If there is only one DS18B20 over a 1-Wire bus, it just need to skip ROM search. The controller just need issue a skip ROM, convert, and read scratch pad command. 

DS18B20 Memory Map

DS18B20 Temperature Data Format

Scratchpad contains 9 bytes of data. Converted temperature consists of two bytes, BYTE 0 and BYTE 1 of device's scratch-pad. It contains sign, decimal temperature data, and fraction in degree Celsius.

To interface with this 1-Wire device, the micro-controller have to prepare a correct timing for its I/O as follow.

DS18B20 Initialization Timing

 

DS18B20 Read/Write Time Slot

This device has two command categories, ROM Commands and Function Commands. ROM commands are not use as the micro-controller just need to get the temperature from a single thermometer on 1-wire bus. 

DS18B20 ROM Commands

1. Search ROM [F0h]
2. Read ROM [33h]
3. Match ROM [55h]
4. Skip ROM [CCh]
5. Alarm Search [ECh]

DS18B20 Function Commands

1. Convert Temperature [44h]
2. Write Scratchpad [4Eh]
3. Read Scracthpad [BEh]
4. Copy Scratchpad [48h]
5. Recall EEPROM [B8h]
6. Read Power Supply [B4h]

For full explanation for these command please read the device's data sheet.

XC8 Programming

I use the MPLABX IDE 1.51 with XC8 v2.36 C compiler for PIC micro-controller. I need to write about five C functions to interact with this device. These C functions I followed an application note published by ANALOG DEVICES, "Interfacing the DS18X20/DS1822 1-Wire® Temperature Sensor in a Microcontroller Environment". This example use the 8051 micro-controller.

Simulating Program In Proteus

Multiplexing Seven-Segment Display

 

PIC16F84A In P-DIP Package From Microchip Website

In this example I use a simple 8-bit micro-controller PIC16F84A to get the temperature from this device. The output device is a simple 4-digit common cathode multiplexing display. For the first conversion time, the thermometer yield 85 degree Celsius but I skip this data.

 
#include<xc.h>
 
 
// PIC16F84A Configuration Bit Settings
 
// CONFIG
#pragma config FOSC = XT        // Oscillator Selection bits (XT oscillator)
#pragma config WDTE = OFF       // Watchdog Timer (WDT disabled)
#pragma config PWRTE = OFF      // Power-up Timer Enable bit (Power-up Timer is disabled)
#pragma config CP = OFF         // Code Protection bit (Code protection disabled)
 
 
#define _XTAL_FREQ 4000000
 
#define DQ RA4
#define DQ_DIR TRISA4
 
unsigned char ow_reset(void){
    unsigned char presence;
    DQ_DIR=0;
    DQ=0;
    __delay_us(480);
    DQ=1;
    __delay_us(70);
    DQ_DIR=1;
    presence=DQ;
    __delay_us(425);
    return presence;
}
 
unsigned char readBit(void){
    DQ_DIR=0;
    DQ=0;
    DQ_DIR=1;
    __delay_us(15);
    return DQ;
}
 
void writeBit(char bitVal){
    DQ_DIR=0;
    DQ=0;
    if(bitVal==1) DQ=1;
    __delay_us(104);
    DQ=1;
}
 
unsigned char readByte(void){
    unsigned char i;
    unsigned char value=0;
    for(i=0;i<8;i++){
        if(readBit()) value|=0x01<<i;
        __delay_us(96);
    }
    return value;
}
 
void writeByte(char val){
    unsigned char i;
    unsigned char temp;
    for(i=0;i<8;i++){
        temp=val>>i;
        temp&=0x01;
        writeBit(temp);
    }
    __delay_us(104);
}
 
 
void main(void){
    unsigned char LSB,MSB;
    unsigned char get[9];
    unsigned int temp;
    //unsigned char ssd[16]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F,0x77,0x7C,0x39,0x5E,0x79,0x71};
    unsigned char ssd[10]={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};
    PORTB=0x00;
    TRISB=0x00;
    PORTA=0x00;
    TRISA=0x00;
    ow_reset();
    writeByte(0xCC);
    writeByte(0x44);
    __delay_us(104);
    ow_reset();
    writeByte(0xCC);
    writeByte(0xBE);
    for(int i=0;i<9;i++) get[i]=readByte();
 
    __delay_ms(800);
    while(1){
        ow_reset();
        writeByte(0xCC);
        writeByte(0x44);
        __delay_us(104);
        ow_reset();
        writeByte(0xCC);
        writeByte(0xBE);
        for(int i=0;i<9;i++) get[i]=readByte();
        MSB=get[1];
        LSB=get[0];
        temp=(MSB<<8)+LSB;
        temp>>=4;
        LSB=0x0F&LSB;
        if(LSB^0x08) LSB=0;
        else LSB=5;
        //temp&=0x00FF;
        //PORTB=temp;
        __delay_ms(10);
 
        PORTA=0x00;
        PORTB=ssd[temp/100];
        if(temp/100) RA0=1;
        __delay_ms(5);
 
        PORTA=0x00;
        PORTB=ssd[(temp%100)/10];
        RA1=1;
        __delay_ms(5);
 
        PORTA=0x00;
        PORTB=ssd[temp%10]|0x80;
        RA2=1;
        __delay_ms(5);
 
        PORTA=0x00;
        PORTB=ssd[LSB];
        RA3=1;
        __delay_ms(5);
 
    }
}
 

This program uses half of device program space and about 80% of device data space. Click here to download its source file.

RAM ROM Usage In PIC16F84A

CCS PICC provide a touch sensor library similar to the DS18B20. MikroC also has a 1-Wire library for its supported devices. Arduino has a lot of libraries for various type of sensors including this device.