DS18B20 16-bit sign-extended two's complement to degrees C

[Freddie]

I'm using the DS18B20 1-wire temperature sensor in the 12 bit resolution mode (default mode). I have no problems communicating with the device, but I do not understand how to convert the 16-bit sign-extended two's complement to degrees C. I searched this forum and using Google but could not find any help for the DS18B20. (I did find tons of stuff for the DS1820 though).

If someone understands 16-bit sign-extended two's complement and could post a CCS code snip I'd appreciate it. Thanks.

Here are some example conversions from the data sheet, but they don't step through how they get the conversion.
07D0 hex = +125C
0191 hex = +25.0625C
0008 hex = +0.5C
0000 hex = 0C
FFF8 hex = -0.5C
FE6F hex = -25.0625C

[newguy]

I think I can help you out, as I've used quite a few of the DS temp sensors.

Think of the 3 most significant nibbles (nibble = 4 bits) as the whole portion of the temperature. The least significant nibble is the fractional portion of the temperature (.5, .25, etc.).

So, examining the values from the data sheet:

0x07D0 - last nibble (fractional part) 0 = 0x07D = 7 x 16 + 13 = 125. Follow?

0x0191 - last nibble 1 = 0x019 = 1 x 16 + 9 = 25. Now for the fractional part: that last nibble is broken down into four bits. The most significant bit is .5C (half), followed by 0.25C (quarter), then 0.125C (eighth), and the least significant bit of the fractional nibble is 0.0625C (sixteenth). So, 0x0191 = 25C + 1/16C = 25.0625C. Easy, right?

So, 0xFFF8 - last nibble 8 = 0xFFF which is -1C, right? We still add to that the fractional part, 8 = 0.5C, and end up with -0.5C.

Finally, 0xFE6F - last nibble = 0XFE6 = -26C. We add the fractional part, F = 0.5C + 0.25C + 0.125C + 0.0625C = 15/16C = 0.9375C. So -26C + 0.9375C = -25.0625C.

Hope this helps.

-- Mark

[Ttelmah]

[Freddie]

Thanks "newguy".

I got the DS18B20 working and thought I would post the code here so others can use it. It may not be optimized but it does work. As stated in the header, this code uses the CCS 1-wire driver file <touch.c> and assumes one DS18B20 on RB0. The code reads the Serial Number (not necessary for the temperature conversion), performs a Skip ROM temperature read, converts the digital temperature to C and F, and sends data to a PC. My temperature conversion routine is in bold below.

If anyone has more efficient code that converts the DS18B20 16 bit tempertaure to degrees C, please share it here.


//
// DS18B20 Temperature.
// Reads DS18B20 Serial number, performs a skip ROM temperature conversion,
// sends serial number, digital, C and F temperature data to PC serial port.
//
// May not be optimized but it is tested and works. Use at your own risk.
// Future updates to include CRC calculation, specific ROM temperature reads and search ROM.
//
// 16F876A Pin 33 (RB0) connected to DS18B20 Pin2 (DQ).
// 16F876A Pin 33 (RB0) also connect to 4.7K resistor pulled up to +5V.
// 16F876A Pin 25 (RC6/Tx) connected to MAX232 or Max233 which is then connected to PC Parallel Port.
// DS18B20 Pin1(grd) and Pin2(Vdd) connected to ground.
//
// Uses the CCS low level DS 1-wire drivers found in <touch.c>.
//

#include <16F876a.h>

#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=10000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)

#include <touch.c>
#include <string.h>
#include <stdlib.h>


void main() {
int8 buffer[10];
int16 temp_digital;
float temp_c, temp_f;

printf("\r\nWaiting for a device...\r\n");

while (1)
{

//read SN command.
if(touch_present())
{
touch_write_byte(0x33); //read ROM command
buffer[0] = touch_read_byte(); //family code
buffer[1] = touch_read_byte(); //SN1 LSByte
buffer[2] = touch_read_byte(); //SN2
buffer[3] = touch_read_byte(); //SN3
buffer[4] = touch_read_byte(); //SN4
buffer[5] = touch_read_byte(); //SN5
buffer[6] = touch_read_byte(); //SN6 MSByte
buffer[7] = touch_read_byte(); //CRC

printf ("S/N READ: %X %X %X %X %X %X %X %X \r\n",buffer[0],buffer[1],buffer[2],buffer[3],buffer[4],buffer[5],buffer[6],buffer[7]);

delay_ms (1000);

} //end if

//read Temperature.
if(touch_present())
{
touch_write_byte(0xCC); //skip ROM
touch_write_byte(0x44); //temperature convert command
output_float(TOUCH_PIN); //strong pullup
delay_ms(1000);

touch_present();
touch_write_byte(0xCC); //skip ROM
touch_write_byte(0xBE); //copy scratchpad command

//get scratchpad data
buffer[0] = touch_read_byte();
buffer[1] = touch_read_byte();
buffer[2] = touch_read_byte();
buffer[3] = touch_read_byte();
buffer[4] = touch_read_byte();
buffer[5] = touch_read_byte();
buffer[6] = touch_read_byte();
buffer[7] = touch_read_byte();
buffer[8] = touch_read_byte();

printf ("Temp READ: %X %X %X %X %X %X %X %X %X \r\n",buffer[0],buffer[1],buffer[2],buffer[3],buffer[4],buffer[5],buffer[6],buffer[7],buffer[8]);
temp_digital = make16(buffer[1],buffer[0]);
printf( "temp_digital= %LX \r\n", temp_digital);

if (temp_digital >= 0x800) //temperture is negative
{
temp_c = 0;
//calculate the fractional part
if(temp_digital & 0x0001) temp_c += 0.06250;
if(temp_digital & 0x0002) temp_c += 0.12500;
if(temp_digital & 0x0004) temp_c += 0.25000;
if(temp_digital & 0x0008) temp_c += 0.50000;

//calculate the whole number part
temp_digital = (temp_digital >> 4) & 0x00FF;
temp_digital = temp_digital - 0x0001; //subtract 1
temp_digital = ~temp_digital; //ones compliment
temp_c = temp_c - (float)(temp_digital & 0xFF);
}
else //temperture is positive
{
temp_c = 0;
//calculate the whole number part
temp_c = (temp_digital >> 4) & 0x00FF;
//calculate the fractional part
if(temp_digital & 0x0001) temp_c = temp_c + 0.06250;
if(temp_digital & 0x0002) temp_c = temp_c + 0.12500;
if(temp_digital & 0x0004) temp_c = temp_c + 0.25000;
if(temp_digital & 0x0008) temp_c = temp_c + 0.50000;
} //end if else

printf( "TempC= %7.3f degrees C \r\n", temp_c ); // print temp C
temp_f = (temp_c* 9.00000)/5.00000 + 32.00000;
printf( "TempF= %7.3f degrees F \r\n\r\n", temp_f ); // print temp F
delay_ms (1000);

} //end if

} //end while(1)

} //end main

[Guest_X44]

The datasheet of DS1631 shows that for a digital output of 0xE6F0 equivalent temperature in degrees celsius is -25.0625.

For a digital output of 0xF5E0 the equivalent temperature in degrees celsius is -10.125.



I don't quite get it. For positive temperatures the explanation in this thread seems right. For negatives, I'm at a loss.

Could someone spell out.

[Guest_X44]

I forgot to mention that bit 15 is the sign bit (S). S=0 if temperature is positive, and S=1 if temperature is negative.

Also, bits 3 through 0 are hardwired to 0.

[Neutone]

[jds-pic2]

[Guest]

Freddie,
first thanks the codes, i use ur program and modify a little bit(only change it to PIC16F877A), finally i got the program running, but the temperature is not quit accurate about 12 degree C different, i use a DS1820 sensor. here is the codes:

//
// DS18B20 Temperature.
// Reads DS18B20 Serial number, performs a skip ROM temperature conversion,
// sends serial number, digital, C and F temperature data to PC serial port.
//
// May not be optimized but it is tested and works. Use at your own risk.
// Future updates to include CRC calculation, specific ROM temperature reads and search ROM.
//
// 16F877A Pin 33 (RB0) connected to DS18B20 Pin2 (DQ).
// 16F877A Pin 33 (RB0) also connect to 4.7K resistor pulled up to +5V.
// 16F877A Pin 25 (RC6/Tx) connected to MAX232 or Max233 which is then connected to PC serial Port.
// DS18B20 Pin1(grd) and Pin2(Vdd) connected to ground.
//
// Uses the CCS low level DS 1-wire drivers found in <touch.c>.
//

//#include <16F876a.h>
#include <16F877a.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)

#include <touch.c>
#include <string.h>
#include <stdlib.h>


void main() {
int8 buffer[10];
int16 temp_digital;
float temp_c, temp_f;

printf("\r\nWaiting for a device...\r\n");

while (1)
{

//read SN command.
if(touch_present())
{
touch_write_byte(0x33); //read ROM command
buffer[0] = touch_read_byte(); //family code
buffer[1] = touch_read_byte(); //SN1 LSByte
buffer[2] = touch_read_byte(); //SN2
buffer[3] = touch_read_byte(); //SN3
buffer[4] = touch_read_byte(); //SN4
buffer[5] = touch_read_byte(); //SN5
buffer[6] = touch_read_byte(); //SN6 MSByte
buffer[7] = touch_read_byte(); //CRC

printf ("S/N READ: %X %X %X %X %X %X %X %X \r\n",buffer[0],buffer[1],buffer[2],buffer[3],buffer[4],buffer[5],buffer[6],buffer[7]);

delay_ms (1000);

} //end if

//read Temperature.
if(touch_present())
{
touch_write_byte(0xCC); //skip ROM
touch_write_byte(0x44); //temperature convert command
output_float(TOUCH_PIN); //strong pullup
delay_ms(1000);

touch_present();
touch_write_byte(0xCC); //skip ROM
touch_write_byte(0xBE); //copy scratchpad command

//get scratchpad data
buffer[0] = touch_read_byte();
buffer[1] = touch_read_byte();
buffer[2] = touch_read_byte();
buffer[3] = touch_read_byte();
buffer[4] = touch_read_byte();
buffer[5] = touch_read_byte();
buffer[6] = touch_read_byte();
buffer[7] = touch_read_byte();
buffer[8] = touch_read_byte();

printf ("Temp READ: %X %X %X %X %X %X %X %X %X \r\n",buffer[0],buffer[1],buffer[2],buffer[3],buffer[4],buffer[5],buffer[6],buffer[7],buffer[8]);
temp_digital = make16(buffer[1],buffer[0]);
printf( "temp_digital= %LX \r\n", temp_digital);

if (temp_digital >= 0x800) //temperture is negative
{
temp_c = 0;
//calculate the fractional part
if(temp_digital & 0x0001) temp_c += 0.06250;
if(temp_digital & 0x0002) temp_c += 0.12500;
if(temp_digital & 0x0004) temp_c += 0.25000;
if(temp_digital & 0x0008) temp_c += 0.50000;

//calculate the whole number part
temp_digital = (temp_digital >> 4) & 0x00FF;
temp_digital = temp_digital - 0x0001; //subtract 1
temp_digital = ~temp_digital; //ones compliment
temp_c = temp_c - (float)(temp_digital & 0xFF);
}
else //temperture is positive
{
temp_c = 0;
//calculate the whole number part
temp_c = (temp_digital >> 4) & 0x00FF;
//calculate the fractional part
if(temp_digital & 0x0001) temp_c = temp_c + 0.06250;
if(temp_digital & 0x0002) temp_c = temp_c + 0.12500;
if(temp_digital & 0x0004) temp_c = temp_c + 0.25000;
if(temp_digital & 0x0008) temp_c = temp_c + 0.50000;
} //end if else

printf( "TempC= %7.3f degrees C \r\n", temp_c ); // print temp C
temp_f = (temp_c* 9.00000)/5.00000 + 32.00000;
printf( "TempF= %7.3f degrees F \r\n\r\n", temp_f ); // print temp F
delay_ms (1000);

} //end if

} //end while(1)

} //end main


Jay

[jds-pic]

[hayri]

My convertion code:

[hayri]

note:


you can call converttemp:

int8 buff[9];
...
...
...

buff[0] = DS1820_Read_FIRST_TEMPERATURE_Byte();
buff[1] = DS1820_Read_SECOND_TEMPERATURE_Byte();
//Read other 7 bytes

ConvertTemp(buff[1],buff[0]);

[jds-pic]

[hayri]

every corrections or suggestions are welcome

infact i'm using routines like you decribe. at least since i switched pic18f452. when i using pic16f628A and %100 rom space i have to write something like that. and when i saw this topic i post this one because it is better to understand whats going on with this one.

stucture likes jds-pic's suggestion is easy to use and more self-describted.
i also suggest this approach. For ds1820 raw reading routines please search the forum. you will find!