Hi guys I managed to make this code work on dallas 1wire sensor, but I have a couple doubts regarding how data is accesed from the sensor. I obviously read the datasheet from dallas.
1) I know there’s a 64 bit rom code, 8 bit crc, 48-bit serial, and family code.
I’m confusing myself where’s this data stored is it on the scratchpad.
I know the scratchpad has 9 bytes of data going from
Byte 0 Temperature LSB
1 Temperature MSB
Byte 2 Th Register
3 TL Register
Byte 4 Configuration Register
6 Reserved
7 Reserved
8 CRC
So to read the serial number this is used in the code example. Why does looping from 0 to 8 show up the hex values from the sensor, if addr as an array it’s initialized with nothing inside ?
Serial.print(“ROM =”);
for( i = 0; i < 8; i++) {
Serial.write(’ ');
Serial.print(addr*, HEX);*
2)
- ds.select(addr); *
- ds.write(0xBE); // Read Scratchpad*
I don’t understand ds.select(addr) where its pointing at. Addr was an array I understand that. Command BE it’s instructed to start reading values coming out of the sensor?
3)
* *Serial.print(" Data = "); Serial.print(present, HEX); Serial.print(" "); for ( i = 0; i < 9; i++) { // we need 9 bytes data[i] = ds.read(); Serial.print(data[i], HEX); Serial.print(" ");* * - This makes sense to me there’s a for loop to read into data array data coming from byte 0 to 9 . So ds.read() it’s the function from onewire instructing the DS object defined to read data from the registers ?*
4)
Here I’m completely lost , somehow, I understand datap[1] contains byte 1 from sensor which in turn contains MS Byte and data[0] Lsb , but why is data[1] bit shifted left 8 positions t that way it’s been left with 00000000 and bitwise or with data[0] ending with just an 8 bit value. - Then I’m lost with if (type_s) and so on. type_s evaluates if it’s DS18B20 family . So if it’s 1 if it’s true goes on…*
``` - int16_t raw = (data[1] << 8) | data[0];
if (type_s) {
raw = raw << 3; // 9 bit resolution default
if (data[7] == 0x10) {
// “count remain” gives full 12 bit resolution
raw = (raw & 0xFFF0) + 12 - data[6];
}
} else {
byte cfg = (data[4] & 0x60);
// at lower res, the low bits are undefined, so let’s zero them
if (cfg == 0x00) raw = raw & ~7; // 9 bit resolution, 93.75 ms
else if (cfg == 0x20) raw = raw & ~3; // 10 bit res, 187.5 ms
else if (cfg == 0x40) raw = raw & ~1; // 11 bit res, 375 ms
//// default is 12 bit resolution, 750 ms conversion time
}*
* *Complete Code - Dallas DS18B20* *
*OneWire ds(10); // on pin 10 (a 4.7K resistor is necessary)
void setup(void) {
Serial.begin(9600);
}
void loop(void) {
byte i;
byte present = 0;
byte type_s;
byte data[12];
byte addr[8];
float celsius, fahrenheit;
if ( !ds.search(addr)) {
Serial.println(“No more addresses.”);
Serial.println();
ds.reset_search();
delay(250);
return;
}
Serial.print(“ROM =”);
for( i = 0; i < 8; i++) {
Serial.write(’ ');
Serial.print(addr[i], HEX);
}
if (OneWire::crc8(addr, 7) != addr[7]) {
Serial.println(“CRC is not valid!”);
return;
}
Serial.println();
// the first ROM byte indicates which chip
switch (addr[0]) {
case 0x10:
Serial.println(" Chip = DS18S20"); // or old DS1820
type_s = 1;
break;
case 0x28:
Serial.println(" Chip = DS18B20");
type_s = 0;
break;
case 0x22:
Serial.println(" Chip = DS1822");
type_s = 0;
break;
default:
Serial.println(“Device is not a DS18x20 family device.”);
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
Serial.print(" Data = “);
Serial.print(present, HEX);
Serial.print(” “);
for ( i = 0; i < 9; i++) { // we need 9 bytes
data[i] = ds.read();
Serial.print(data[i], HEX);
Serial.print(” “);
}
Serial.print(” CRC=");
Serial.print(OneWire::crc8(data, 8), HEX);
Serial.println();
// Convert the data to actual temperature
// because the result is a 16 bit signed integer, it should
// be stored to an “int16_t” type, which is always 16 bits
// even when compiled on a 32 bit processor.
int16_t raw = (data[1] << 8) | data[0];
if (type_s) {
raw = raw << 3; // 9 bit resolution default
if (data[7] == 0x10) {
// “count remain” gives full 12 bit resolution
raw = (raw & 0xFFF0) + 12 - data[6];
}
} else {
byte cfg = (data[4] & 0x60);
// at lower res, the low bits are undefined, so let’s zero them
if (cfg == 0x00) raw = raw & ~7; // 9 bit resolution, 93.75 ms
else if (cfg == 0x20) raw = raw & ~3; // 10 bit res, 187.5 ms
else if (cfg == 0x40) raw = raw & ~1; // 11 bit res, 375 ms
//// default is 12 bit resolution, 750 ms conversion time
}
celsius = (float)raw / 16.0;
fahrenheit = celsius * 1.8 + 32.0;
Serial.print(" Temperature = “);
Serial.print(celsius);
Serial.print(” Celsius, “);
Serial.print(fahrenheit);
Serial.println(” Fahrenheit");
}*
```
