Rainbow Electronics DS18B20X - Manual (Page 3)

DS18B20X
3 of 21
OPERATION – MEASURING TEMPERATURE
The core functionality of the DS18B20X is its direct-to-digital temperature sensor. The resolution of the
temperature sensor is user-configurable to 9, 10, 11, or 12 bits, corresponding to increments of 0.5
°
C,
0.25
°
C, 0.125
°
C, and 0.0625
°
C, respectively. The default resolution at power-up is 12 bit. The
DS18B20X powers-up in a low-power idle state; to initiate a temperature measurement and A-to-D
conversion, the master must issue a Convert T [44h] command. Following the conversion, the resulting
thermal data is stored in the 2-byte temperature register in the scratchpad memory and the DS18B20X
returns to its idle state. If the DS18B20X is powered by an external supply, the master can issue “read
time slots” (see the 1-WIRE BUS SYSTEM section) after the Convert T command and the DS18B20X
will respond by transmitting 0 while the temperature conversion is in progress and 1 when the conversion
is done. If the DS18B20X is powered with parasite power, this notification technique cannot be used
since the bus must be pulled high by a strong pullup during the entire temperature conversion. The bus
requirements for parasite power are explained in detail in the POWERING THE DS18B20X section of
this datasheet.
The DS18B20X output temperature data is calibrated in degrees centigrade; for Fahrenheit applications, a
lookup table or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended
two’s complement number in the temperature register (see Figure 2). The sign bits (S) indicate if the
temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the
DS18B20X is configured for 12-bit resolution, all bits in the temperature register will contain valid data.
For 11-bit resolution, bit 0 is undefined. For 10-bit resolution, bits 1 and 0 are undefined, and for 9-bit
resolution bits 2, 1 and 0 are undefined. Table 2 gives examples of digital output data and the
corresponding temperature reading for 12-bit resolution conversions.
TEMPERATURE REGISTER FORMAT Figure 2
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
LS Byte 2
3
2
2
2
1
2
0
2
-1
2
-2
2
-3
2
-4
bit 15 bit 14 bit 13 bit 12 bit 11 bit 10 bit 9 bit 8
MS Byte S S S S S 2
6
2
5
2
4
TEMPERATURE/DATA RELATIONSHIP Table 2
TEMPERATURE DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+125°C 0000 0111 1101 0000 07D0h
+85°C* 0000 0101 0101 0000 0550h
+25.0625°C 0000 0001 1001 0001 0191h
+10.125°C 0000 0000 1010 0010 00A2h
+0.5°C 0000 0000 0000 1000 0008h
0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1111 1000 FFF8h
-10.125°C 1111 1111 0101 1110 FF5Eh
-25.0625°C 1111 1110 0110 1111 FE6Fh
-55°C 1111 1100 1001 0000 FC90h
*The power-on reset value of the temperature register is +85°C