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 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
This variable must be defined in any project that is using OneWire Library: | Description: | Example : |
---|---|---|
extern sfr sbit bdata OW_Bit; |
OneWire line. | sbit OW_Bit at P2_7_bit; |
Library Routines
Ow_Reset
Prototype |
unsigned short Ow_Reset(); |
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Returns |
|
Description |
Issues OneWire reset signal for DS18x20. Parameters :
|
Requires |
Devices compliant with the Dallas OneWire protocol. Global variable |
Example |
// Issue Reset signal on One-Wire Bus Ow_Reset(); |
Ow_Read
Prototype |
unsigned short Ow_Read(); |
---|---|
Returns |
Data read from an external device over the OneWire bus. |
Description |
Reads one byte of data via the OneWire bus. |
Requires |
Devices compliant with the Dallas OneWire protocol. Global variable |
Example |
// Read a byte from the One-Wire Bus unsigned short read_data; ... read_data = Ow_Read(); |
Ow_Write
Prototype |
void Ow_Write(char par); |
---|---|
Returns |
Nothing. |
Description |
Writes one byte of data via the OneWire bus. Parameters :
|
Requires |
Devices compliant with the Dallas OneWire protocol. Global variable |
Example |
// 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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