OneWire Library

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 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.

Ow_Reset

Prototype	sub function Ow_Reset(dim byref port as word, dim pin as word) as word
Description	Issues OneWire reset signal for DS18x20.
Parameters	`port:` OneWire bus port `pin:` OneWire bus pin
Returns	`0` if the device is present `1` if the device is not present
Requires	Devices compliant with the Dallas OneWire protocol.
Example	' Issue Reset signal on One-Wire Bus connected to pin RF6 Ow_Reset(PORTF, 6)
Notes	None.

Ow_Read

Prototype	sub function Ow_Read(dim byref port as word, dim pin as word) as byte
Description	Reads one byte of data via the OneWire bus.
Parameters	`port:` OneWire bus port `pin:` OneWire bus pin
Returns	Data read from an external device over the OneWire bus.
Requires	Devices compliant with the Dallas OneWire protocol.
Example	' Read a byte from the One-Wire Bus connected to pin RF6 dim read_data as byte ... read_data = Ow_Read(PORTF, 6)
Notes	None.

Ow_Write

Prototype	sub procedure Ow_Write(dim byref port as word, dim pin, data_ as word)
Description	Writes one byte of data via the OneWire bus.
Parameters	`port:` OneWire bus port `pin:` OneWire bus pin `data_:` data to be written
Returns	Nothing.
Requires	Devices compliant with the Dallas OneWire protocol.
Example	' Send a byte to the One-Wire Bus connected to pin RF6 Ow_Write(PORTF, 6, 0xCC)
Notes	None.

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
dim LCD_RS as sbit at LATB4_bit
dim LCD_EN as sbit at LATB6_bit
dim LCD_D4 as sbit at LATD4_bit
dim LCD_D5 as sbit at LATD5_bit
dim LCD_D6 as sbit at LATD6_bit
dim LCD_D7 as sbit at LATD7_bit

dim LCD_RS_Direction as sbit at TRISB4_bit
dim LCD_EN_Direction as sbit at TRISB6_bit
dim LCD_D4_Direction as sbit at TRISD4_bit
dim LCD_D5_Direction as sbit at TRISD5_bit
dim LCD_D6_Direction as sbit at TRISD6_bit
dim LCD_D7_Direction as sbit at TRISD7_bit
' End LCD module connections

'  Set TEMP_RESOLUTION to the corresponding resolution of used DS18x20 sensor:
'  18S20: 9  (default settingcan be 9,10,11,or 12)
'  18B20: 12
const TEMP_RESOLUTION as byte = 9

dim text as char[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:
  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
    '--- 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) << 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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