Problem with the code for DS18S20 sensor.

Hello

I´ve got a problem with a DS18S20 sensor. I´ve used a code from an already existing project but it were built for an DS18B20 sensor.
When I run the code it works but I cant get the right value from the sensor. I run the sensor in a room (21 C degrees) but the sensor says 26 C degrees. Please someone help me. I think I must have missed something in the code that only works for a DS18B20. Hope you understand my bad english :slight_smile:

#define TEMP_PIN 5 //

void OneWireReset(int Pin);
void OneWireOutByte(int Pin, byte d);
byte OneWireInByte(int Pin);

void setup() {
digitalWrite(TEMP_PIN, LOW);
pinMode(TEMP_PIN, INPUT); // sets the digital pin as input (logic 1)
Serial.begin(9600);
delay(100);
Serial.print(“Temperatur:\n”);
}

void loop(){
int HighByte, LowByte, TReading, SignBit, Tc_100, Whole, Fract;

OneWireReset(TEMP_PIN);
OneWireOutByte(TEMP_PIN, 0xcc);
OneWireOutByte(TEMP_PIN, 0x44); // perform temperature conversion, strong pullup for one sec

OneWireReset(TEMP_PIN);
OneWireOutByte(TEMP_PIN, 0xcc);
OneWireOutByte(TEMP_PIN, 0xbe);

LowByte = OneWireInByte(TEMP_PIN);
HighByte = OneWireInByte(TEMP_PIN);
TReading = (HighByte << 8) + LowByte;
SignBit = TReading & 0x8000; // test most sig bit
if (SignBit) // negative
{
TReading = (TReading ^ 0xffff) + 1; // 2’s comp
}
Tc_100 = ((TReading*100/2)); // multiply by 0.5

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

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

Serial.print(Fract);

Serial.print("\n");
delay(5000);
}

void OneWireReset(int Pin) // reset. Should improve to act as a presence pulse
{
digitalWrite(Pin, LOW);
pinMode(Pin, OUTPUT); // bring low for 500 us
delayMicroseconds(500);
pinMode(Pin, INPUT);
delayMicroseconds(500);
}

void OneWireOutByte(int Pin, byte d) // output byte d (least sig bit first).
{
byte n;

for(n=8; n!=0; n–)
{
if ((d & 0x01) == 1) // test least sig bit
{
digitalWrite(Pin, LOW);
pinMode(Pin, OUTPUT);
delayMicroseconds(5);
pinMode(Pin, INPUT);
delayMicroseconds(60);
}
else
{
digitalWrite(Pin, LOW);
pinMode(Pin, OUTPUT);
delayMicroseconds(60);
pinMode(Pin, INPUT);
}

d=d>>1; // now the next bit is in the least sig bit position.
}

}

byte OneWireInByte(int Pin) // read byte, least sig byte first
{
byte d, n, b;

for (n=0; n<8; n++)
{
digitalWrite(Pin, LOW);
pinMode(Pin, OUTPUT);
delayMicroseconds(5);
pinMode(Pin, INPUT);
delayMicroseconds(5);
b = digitalRead(Pin);
delayMicroseconds(50);
d = (d >> 1) | (b<<7); // shift d to right and insert b in most sig bit position
}
return(d);
}

//Will_jr

Temperature can fluctuate locally although 5 degrees is quite much. Does the IC feel hot? Or is it near a hot powersupply, laptopFan , ...?

There may be a problem with the precision (number of bits) as the 18S20 supports only 9 bits and the 18B20 supports 9..12 bit. Check the datasheet to see if the formulas and bit positions are identical. As far as I recall they are, and the code looks OK.

The Dallas Temperature Library support both IC's. Install it and you would have an alternative reference. - http://milesburton.com/index.php/Dallas_Temperature_Control_Library

Finally you can make displaying the Temp easier with a float var

float Temperature = TReading * 0.5; Serial.println(Temperature, 1); // print with 1 bit precision

give it a try