The OneWire library provides routines for communication via the Dallas OneWire protocol, for example 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 also has 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.

  Important :

• Oscillator frequency Fosc needs to be at least 4MHz in order to use the routines with Dallas digital thermometers.
• This library implements time-based activities, so interrupts need to be disabled when using OneWire library.

Library Routines

• Ow_Reset
• Ow_Read
• Ow_Write

Ow_Reset

// Issue Reset signal on One-Wire Bus connected to pin RF6
Ow_Reset(PORTF, 6);

Ow_Read

// Read a byte from the One-Wire Bus connected to pin RF6
var read_data : byte;
...
read_data := Ow_Read(PORTF, 6);

Ow_Write

// Send a byte to the One-Wire Bus connected to pin RF6
Ow_Write(PORTF, 6, 0xCC);

Library Example

This example reads the temperature using DS18x20 connected to pin RF6. After reset, MCU obtains temperature from the sensor and prints it on the Lcd. Be sure to set Fosc appropriately in your project, to pull-up RF6 line and to turn off the PORTF leds.

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program OneWire;

// LCD module connections
var LCD_RS : sbit at LATB4_bit;
var LCD_EN : sbit at LATB6_bit;
var LCD_D4 : sbit at LATD4_bit;
var LCD_D5 : sbit at LATD5_bit;
var LCD_D6 : sbit at LATD6_bit;
var LCD_D7 : sbit at LATD7_bit;

var LCD_RS_Direction : sbit at TRISB4_bit;
var LCD_EN_Direction : sbit at TRISB6_bit;
var LCD_D4_Direction : sbit at TRISD4_bit;
var LCD_D5_Direction : sbit at TRISD5_bit;
var LCD_D6_Direction : sbit at TRISD6_bit;
var LCD_D7_Direction : sbit at TRISD7_bit;
// End LCD module connections

//  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 : byte = 9;

var text : array[9] of char;
    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

  ADPCFG := 0;                                      // Configure AN pins as digital I/O

  text := '000.0000';
  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(PORTF, 6);                           // Onewire reset signal
      Ow_Write(PORTF, 6, 0xCC);                     // Issue command SKIP_ROM
      Ow_Write(PORTF, 6, 0x44);                     // Issue command CONVERT_T
      Delay_us(120);

      Ow_Reset(PORTF, 6);
      Ow_Write(PORTF, 6, 0xCC);                     // Issue command SKIP_ROM
      Ow_Write(PORTF, 6, 0xBE);                     // Issue command READ_SCRATCHPAD
      
      temp :=  Ow_Read(PORTF, 6);
      temp := (Ow_Read(PORTF, 6) 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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