OneWire Library

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 8MHz 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.

Ow_Reset

Prototype	sub function Ow_Reset(dim byref port as byte, pin as byte) as byte
Returns	`0` if the device is present `1` if the device is not present
Description	Issues OneWire reset signal for DS18x20. Parameters : `port:` OneWire bus port `pin:` OneWire bus pin
Requires	Works with Dallas DS1820 temperature sensor only.
Example	To reset the DS1820 that is connected to the RB2 pin: Ow_Reset(PORTB, 2)

Ow_Read

Prototype	sub function Ow_Read(dim byref port as byte, dim pin as byte) as byte
Returns	Data read from an external device over the OneWire bus.
Description	Reads one byte of data via the OneWire bus. Parameters : `port:` OneWire bus port `pin:` OneWire bus pin
Requires	Works with Dallas DS1820 temperature sensor only.
Example	tmp = Ow_Read(PORTB, 2)

Ow_Write

Prototype	sub procedure Ow_Write(dim byref port as byte, dim pin, par as byte)
Returns	Nothing.
Description	Writes one byte of data via the OneWire bus. Parameters : `port:` OneWire bus port `pin:` OneWire bus pin `par:` data to be written
Requires	Works with Dallas DS1820 temperature sensor only.
Example	Ow_Write(PORTB, 2, 0xCC)

Library Example

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

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program OneWire

' LCD module connections
dim LCD_RS as sbit at PORTD2_bit
    LCD_EN as sbit at PORTD3_bit
    LCD_D4 as sbit at PORTD4_bit
    LCD_D5 as sbit at PORTD5_bit
    LCD_D6 as sbit at PORTD6_bit
    LCD_D7 as sbit at PORTD7_bit

    LCD_RS_Direction as sbit at DDD2_bit
    LCD_EN_Direction as sbit at DDD3_bit
    LCD_D4_Direction as sbit at DDD4_bit
    LCD_D5_Direction as sbit at DDD5_bit
    LCD_D6_Direction as sbit at DDD6_bit
    LCD_D7_Direction as sbit at DDD7_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 as byte = 9


dim text as string[9]
    temp as word

sub procedure Display_Temperature( dim temp2write as word )
const RES_SHIFT = TEMP_RESOLUTION - 8

dim temp_whole as byte
    temp_fraction as word

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

    ' extract temp_whole
    temp_whole = word(temp2write >> RES_SHIFT)

    ' convert temp_whole to characters
    if ( temp_whole div 100 ) then
      text[0] = temp_whole div 100  + 48
    else
      text[0] = "0"
    end if

    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 << (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 sub

main:
  text = "000.0000"
'  UART1_Init(9600)
  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
  while TRUE
    '--- perform temperature reading
    Ow_Reset(PORTB, 2)                                   ' Onewire reset signal
    Ow_Write(PORTB, 2, 0xCC)                             ' Issue command SKIP_ROM
    Ow_Write(PORTB, 2, 0x44)                             ' Issue command CONVERT_T
    Delay_us(120)

    Ow_Reset(PORTB, 2)
    Ow_Write(PORTB, 2, 0xCC)                             ' Issue command SKIP_ROM
    Ow_Write(PORTB, 2, 0xBE)                             ' Issue command READ_SCRATCHPAD

    temp =  Ow_Read(PORTB, 2)
    temp = (Ow_Read(PORTB, 2) << 8) + temp

    '--- Format and display result on Lcd

    Display_Temperature(temp)

    Delay_ms(520)
  wend
end.

HW Connection

Example of DS1820 connection

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