Datasheet

DS18B20
4 of 22
Figure 2. Temperature Register Format
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
S = SIGN
Table 1. Temperature/Data Relationship
TEMPERATURE (
°
C)
DIGITAL OUTPUT
(BINARY)
DIGITAL OUTPUT
(HEX)
[phone] 0000 07D0h
+85* 0000 0101 0101 0000 0550h
+25.0625 0000 0001 1001 0001 0191h
+10.125 0000 0000 1010 0010 00A2h
+0.5 0000 0000 0000 1000 0008h
0 0000 0000 0000 0000 0000h
-0.5 1111 1111 1111 1000 FFF8h
-10.125 1111 1111 0101 1110 FF5Eh
-25.0625 1111 1110 0110 1111 FE6Fh
-55 1111 1100 1001 0000 FC90h
*The power-on reset value of the temperature register is +85°C.
OPERATION—ALARM SIGNALING
After the DS18B20 performs a temperature conversion, the temperature value is compared to the user-
defined two’s complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see Figure 3).
The sign bit (S) indicates if the value is positive or negative: for positive numbers S = 0 and for negative
numbers S = 1. The T
H
and T
L
registers are nonvolatile (EEPROM) so they will retain data when the
device is powered down. T
H
and T
L
can be accessed through bytes 2 and 3 of the scratchpad as explained
in the Memory section.
Figure 3. T
H
and T
L
Register Format
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
Only bits 11 through 4 of the temperature register are used in the T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers. If the measured temperature is lower than or equal to T
L
or higher than or equal to T
H
,
an alarm condition exists and an alarm flag is set inside the DS18B20. This flag is updated after every
temperature measurement; therefore, if the alarm condition goes away, the flag will be turned off after the
next temperature conversion.