Arduino and the DS18S20

Arduino and the DS18S20 (in parasite power mode)

Environment:

• Ubuntu 8.04
• Arduino 0012 software
• OneWire library (unzip to arduino-0012/hardware/libraries)
  
• FTDI TTL-232R-5V USB programming cable
• BBB (Arduino clone)
• Dallas DS18S20 temperature sensor
  

Schematic:

• 5V is connected to 4.7K resistor
• Resistor is connected to DS18S20 center pin (data bus)
• Arduino pin 10 is connected to DS18S20 center pin (data bus)
• Ground is connected to DS18S20 pin 1
• Additional sensor can be added in parallel to the shown sensor, with data pin and ground
  

Code:
(mainly from http://www.arduino.cc/playground/Learning/OneWire)

#include <OneWire.h>

// DS18S20 Temperature chip i/o

OneWire ds(10); // on pin 10

void setup(void) {
// initialize inputs/outputs
// start serial port
Serial.begin(9600);
}

void loop(void) {
byte i;
byte present = 0;
byte data[12];
byte addr[8];
int HighByte, LowByte, TReading, SignBit, Tc_100, Whole, Fract;

if ( !ds.search(addr)) {
Serial.print("No more sensors\n");
ds.reset_search();
return;
}

Serial.print("Sensor ID=");
for( i = 0; i < 8; i++) {
Serial.print(addr[i], HEX);
Serial.print(" ");
}

if ( OneWire::crc8( addr, 7) != addr[7]) {
Serial.print("CRC is not valid!\n");
return;
}

if ( addr[0] != 0x10) {
Serial.print("Device is not a DS18S20 family device.\n");
return;
}

ds.reset();
ds.select(addr);
ds.write(0x44,1); // start conversion, with parasite power on at the end

delay(1000); // maybe 750ms is enough, maybe not
// we might do a ds.depower() here, but the reset will take care of it.

present = ds.reset();
ds.select(addr);
ds.write(0xBE); // Read Scratchpad

for ( i = 0; i < 9; i++) { // we need 9 bytes
data[i] = ds.read();
}

LowByte = data[0];
HighByte = data[1];
TReading = (HighByte << 8) + LowByte;
SignBit = TReading & 0x8000; // test most sig bit
if (SignBit) // negative
{
TReading = (TReading ^ 0xffff) + 1; // 2's comp
}
// multiply by (100 * 0.0625) or 6.25 for DS18B20
// Tc_100 = (6 * TReading) + TReading / 4;
// multiply by (100 * 0.5) or 50 for DS18S20
Tc_100 = (50 * TReading);

Whole = Tc_100 / 100; // separate off the whole and fractional portions
Fract = Tc_100 % 100;

Serial.print(" Temp=");
if (SignBit) // If its negative
{
Serial.print("-");
}
Serial.print(Whole);
Serial.print(".");
if (Fract < 10)
{
Serial.print("0");
}
Serial.print(Fract);

Serial.print("\n");
}