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
unsigned short read_data;
...
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.

// LCD module connections
sbit LCD_RS at P2_0_bit;
sbit LCD_EN at P2_1_bit;

sbit LCD_D4 at P2_2_bit;
sbit LCD_D5 at P2_3_bit;
sbit LCD_D6 at P2_4_bit;
sbit LCD_D7 at P2_5_bit;
// End LCD module connections

// OneWire pinout
sbit OW_Bit 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 unsigned short TEMP_RESOLUTION = 9;

char *text = "000.0000";
unsigned temp;

void Display_Temperature(unsigned int temp2write) {
  const unsigned short RES_SHIFT = TEMP_RESOLUTION - 8;
  char temp_whole;
  unsigned int temp_fraction;

  // check if temperature is negative
  if (temp2write & 0x8000) {
    text[0] = '-';
    temp2write = ~temp2write + 1;
  }

  // extract temp_whole
  temp_whole = temp2write >> RES_SHIFT;

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

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

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

  // convert temp_fraction to characters
  text[4] =  temp_fraction/1000    + 48;         // Extract thousands digit
  text[5] = (temp_fraction/100)%10 + 48;         // Extract hundreds digit
  text[6] = (temp_fraction/10)%10  + 48;         // Extract tens digit
  text[7] =  temp_fraction%10      + 48;         // Extract ones digit

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

void main() {

  Lcd_Init();                                    // Initialize LCD
  Lcd_Cmd(_LCD_CLEAR);                           // Clear LCD
  Lcd_Cmd(_LCD_CURSOR_OFF);                      // Turn cursor off
  Lcd_Out(1, 1, " Temperature:   ");
  // Print degree character, 'C' for Centigrades
  Lcd_Chr(2,13,223);  // different LCD displays have different char code for degree
                      // if you see greek alpha letter try typing 178 instead of 223

  Lcd_Chr(2,14,'C');

  //--- main loop
  do {
    //--- 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);

    Ow_Reset();
    Ow_Write(0xCC);                              // Issue command SKIP_ROM
    Ow_Write(0xBE);                              // Issue command READ_SCRATCHPAD

    temp =  Ow_Read();
    temp = (Ow_Read() << 8) + temp;

    //--- Format and display result on Lcd
    Display_Temperature(temp);

    Delay_ms(500);
  } while (1);
}

HW Connection

Example of DS1820 connection

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