DS1820
021497 5/27
TEMPERATURE MEASURING CIRCUITRY Figure 4
SLOPE ACCUMULATOR
PRESET
PRESET
COUNTER
COUNTER
=0
=0
STOP
INC
COMPARE
TEMPERATURE REGISTER
LOW TEMPERATURE
COEFFICIENT OSCILLATOR
HIGH TEMPERATURE
COEFFICIENT OSCILLATOR
SET/CLEAR
LSB
TEMPERATURE/DATA RELATIONSHIPS Table 1
TEMPERATURE
DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+125
°
C
00000000 11111010
00FA
+25
°
C
00000000 00110010
0032h
+
1
/2
°
C
00000000 00000001
0001h
+0
°
C
00000000 00000000
0000h
–
1
/
2
°
C
11111111 11111111
FFFFh
–25
°
C
11111111 11001110
FFCEh
–55
°
C
11111111 10010010
FF92h
OPERATION – ALARM SIGNALING
After the DS1820 has performed a temperature conver-
sion, the temperature value is compared to the trigger
values stored in TH and TL. Since these registers are
8–bit only, the 0.5
°
C bit is ignored for comparison. The
most significant bit of TH or TL directly corresponds to
the sign bit of the 16–bit temperature register. If the
result of a temperature measurement is higher than TH
or lower than TL, an alarm flag inside the device is set.
This flag is updated with every temperature measure-
ment. As long as the alarm flag is set, the DS1820 will
respond to the alarm search command. This allows
many DS1820s to be connected in parallel doing simul-
taneous temperature measurements. If somewhere the
temperature exceeds the limits, the alarming device(s)
can be identified and read immediately without having to
read non–alarming devices.
