The OneWire library provides routines for communication via the Dallas OneWire protocol, e.g. with DS18x20 digital thermometer. OneWire is a Master/Slave protocol, and all communication cabling required is a single wire. OneWire enabled devices should have open collector drivers (with single pull-up resistor) on the shared data line.

Slave devices on the OneWire bus can even get their power supply from data line. For detailed schematic see device datasheet.

Some basic characteristics of this protocol are:

• single master system,
• low cost,
• low transfer rates (up to 16 kbps),
• fairly long distances (up to 300 meters),
• small data transfer packages.

Each OneWire device has also a unique 64-bit registration number (8-bit device type, 48-bit serial number and 8-bit CRC), so multiple slaves can co-exist on the same bus.

Note: Oscillator frequency Fosc needs to be at least 8MHz in order to use the routines with Dallas digital thermometers.

External dependencies of OneWire Library

Library Routines

• Ow_Reset
• Ow_Read
• Ow_Write

Ow_Reset

// Issue Reset signal on One-Wire Bus
Ow_Reset();

Ow_Read

// Read a byte from the One-Wire Bus
var read_data : byte;
...
read_data := Ow_Read();

Ow_Write

// Send a byte to the One-Wire Bus
Ow_Write(0xCC);

Library Example

This example reads the temperature using DS18x20 connected to pin P1.2. After reset, MCU obtains temperature from the sensor and prints it on the Lcd. Make sure to pull-up P1.2 line and to turn off the P1 leds.

program OneWire;

// lcd pinout definition
var LCD_RS : sbit at P2_0_bit;
var LCD_EN : sbit at P2_1_bit;

var LCD_D4 : sbit at P2_2_bit;
var LCD_D5 : sbit at P2_3_bit;
var LCD_D6 : sbit at P2_4_bit;
var LCD_D7 : sbit at P2_5_bit;
// end lcd definition

// OneWire pinout
var OW_Bit : sbit at P1_2_bit;
// end OneWire definition

//  Set TEMP_RESOLUTION to the corresponding resolution of used DS18x20 sensor:
//  18S20: 9  (default setting; can be 9,10,11,or 12)
//  18B20: 12
const TEMP_RESOLUTION = 9;

var text : string[9];
    temp : word;

procedure Display_Temperature( temp2write : word ) ;
const RES_SHIFT = TEMP_RESOLUTION - 8;

var temp_whole : byte;
    temp_fraction : word;

  begin
    text := '000.0000';
    // check if temperature is negative
    if (temp2write and 0x8000) then
      begin
        text[0] := '-';
        temp2write := not temp2write + 1;
      end;

    // extract temp_whole
    temp_whole := word(temp2write shr RES_SHIFT);

    // convert temp_whole to characters
     if ( temp_whole div 100 ) then
         text[0] := temp_whole div 100  + 48
    else
       text[0] := '0';

    text[1] := (temp_whole div 10)mod 10 + 48;         // Extract tens digit
    text[2] :=  temp_whole mod 10    + 48;             // Extract ones digit

    // extract temp_fraction and convert it to unsigned int
    temp_fraction  :=  word(temp2write shl (4-RES_SHIFT));
    temp_fraction := temp_fraction and 0x000F;
    temp_fraction := temp_fraction * 625;


    // convert temp_fraction to characters

    text[4] := word(temp_fraction div 1000)    + 48;         // Extract thousands digit
    text[5] := word((temp_fraction div 100)mod 10 + 48);     // Extract hundreds digit
    text[6] := word((temp_fraction div 10)mod 10  + 48);     // Extract tens digit
    text[7] := word(temp_fraction mod 10)      + 48;         // Extract ones digit

    // print temperature on LCD
    Lcd_Out(2, 5, text);
  end;

  begin

    Lcd_Init();                                     // Initialize LCD
    Lcd_Cmd(_LCD_CLEAR);                            // Clear LCD
    Lcd_Cmd(_LCD_CURSOR_OFF);                       // Turn cursor off
    Lcd_Out(1, 1, ' Temperature:   ');

    Lcd_Chr(2,13,178);    // Print degree character, 'C' for Centigrades
                          // different LCD displays have different char code for degree
    Lcd_Chr(2,14,'C');    // if you see greek alpha letter try typing 178 instead of 223

    //--- main loop
    while TRUE do
      begin
        //--- perform temperature reading
        Ow_Reset();                              // Onewire reset signal
        Ow_Write(0xCC);                          // Issue command SKIP_ROM
        Ow_Write(0x44);                          // Issue command CONVERT_T
        Delay_us(120);                           // added to avoid first false reading
                                                 // upon powering up the device
        Ow_Reset();
        Ow_Write(0xCC);                          // Issue command SKIP_ROM
        Ow_Write(0xBE);                          // Issue command READ_SCRATCHPAD

        temp :=  Ow_Read();
        temp := (Ow_Read() shl 8) + temp;

        //--- Format and display result on Lcd
        Display_Temperature(temp);
        Delay_ms(520);
      end;
  end.

HW Connection

Example of DS1820 connection

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