DS1621
TEMPERATURE/DATA RELATIONSHIPS
Table 2
TEMPERATURE
+125°C
+25°C
+
1/2
°C
+0°C
–
1
/
2
°C
–25°C
–55°C
DIGITAL OUTPUT
(Binary)
01111101 00000000
00011001 00000000
0000000010000000
0000000000000000
11111111 10000000
11100111 00000000
11001001 00000000
DIGITAL OUTPUT
(Hex)
7B00h
1900h
0080h
0000h
FF80h
E700h
C900h
Since data is transmitted over the 2–wire bus MSB first,
temperature data may be written to/read from the
DS1621 as either a single byte (with temperature reso-
lution of 1°C), or as two bytes, the second byte contain-
ing the value of the least significant (0.5°C)bit of the tem-
perature reading, as shown in Table 1. Note that the
remaining 7 bits of this byte are set to all 0’s.
Note that temperature is represented in the DS1621 in
terms of a
1
/
2
°C
LSB, yielding the following 9–bit format:
MSB
1
1
1
0
0
1
1
1
LSB
0
0
0
0
0
0
0
0
Thermostat Control
In its operating mode, the DS1621 functions as a ther-
mostat with programmable hysteresis, as shown in Fig-
ure 3. The thermostat output updates as soon as a tem-
perature conversion is complete.
When the DS1621’s temperature meets or exceeds the
value stored in the high temperature trip register (TH),
the output becomes active, and will stay active until the
temperature falls below the temperature stored in the
low temperature trigger register (TL). In this way, any
amount of hysteresis may be obtained.
The active state for the output is programmable by the
user, so that an active state may either be a logic 1 (V
DD
)
or a logic 0 (0V).
T = –25°C
Higher resolutions may be obtained by reading the tem-
perature, and truncating the 0.5°C bit (the LSB) from the
read value. This value is TEMP_READ. The value left in
the counter may then be read by issuing a READ
COUNTER command. This value is the count remain-
ing (COUNT_REMAIN) after the gate period has
ceased. By loading the value of the slope accumulator
into the count register (using the READ SLOPE com-
mand), this value may then be read, yielding the number
of counts per degree C (COUNT_PER_C) at that tem-
perature. The actual temperature may be then be calcu-
lated by the user using the following:
TEMPERATURE = TEMP_READ – 0.25
)
(COUNT_PER_C
– COUNT_REMAIN)
COUNT_PER_C
THERMOSTAT OUTPUT OPERATION
Figure 3
DQ (Thermostat output, Active=High)
TL
TH
T (°C)
021497 4/14
ETC [ ETC ]
