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DS1721S

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DS1721
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OPERATION-Measuring Temperature
The core of DS1721 functionality is its direct-to-digital temperature sensor. The DS1721 measures
temperature through the use of an on-chip temperature measurement technique with an operation range
from -55°C to +125°C. The device can be configured to perform continuous conversions with the most
recent result being stored in the thermometer register. The device can also be configured to perform a
single conversion, store the result, and return to a standby mode. Regardless of the mode used, the digital
temperature is retrieved from the temperature register using the Read Temperature (AAh) command, as
described in detail in the “Command Set” section. The DS1721 power-up default has the sensor set to
automatically perform 12-bit conversions continuously once the Start Convert T (51h) command is
issued. Details on how to change the settings after power-up are contained in the “OPERATION-
Programming” section.
The resolution of the temperature conversion can be configured as 9, 10, 11, or 12 bits, with 12-bit
readings as the default state. This equates to a temperature resolution of 0.5°C, 0.25°C, 0.125°C, or
0.0625°C. Following each conversion thermal data is stored in the thermometer register in two’s
complement format; the information can be retrieved over the 2-wire interface by issuing a Read
Temperature (AAh) command. Table 2 describes the exact relationship of output data to measured
temperature. The table assumes the DS1721 is configured for 12-bit resolution; if the device is configured
in a lower resolution mode, insignificant bits will contain zeros. The data is transmitted serially over the
2-wire serial interface, MSb first. The MSb of the temperature register contains the “sign” (S) bit,
denoting whether the temperature is positive or negative. For Fahrenheit usage, a lookup table or
conversion routine must be used.
TEMPERATURE/DATA RELATIONSHIPS Table 2
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb (UNIT = °C) LSb
2
-1
2
-2
2
-3
2
-4
0 0 0 0 LSB
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+125°C 0111 1101 0000 0000 7D00h
+25.0625°C 0001 1001 0001 0000 1910h
+10.125°C 0000 1010 0010 0000 0A20h
+0.5°C 0000 0000 1000 0000 0080h
+0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1000 0000 FF80h
-10.125°C 1111 0101 1110 0000 F5E0h
-25.0625°C 1110 0110 1111 0000 E6F0h
-55°C 1100 1001 0000 0000 C900h
OPERATION-Thermostat Control
In its operating mode, the DS1721 functions as a thermostat with programmable hysteresis, as shown in
Figure 2. The thermostat output updates as soon as a temperature conversion is complete. When the
DS1721’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 is equal to or below the temperature
stored in the low temperature trigger register (TL). In this way, any amount of hysteresis may be
obtained.
DS1721
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The active state for the totem-pole output is programmable by the user. The power-up default of the
DS1721 has TH=80°C, TL=75°C, and the output state active high. Refer to the “OPERATION-
Programming” section for instructions in adjusting the thermostat setpoints and T
COM
active state.
THERMOSTAT OUTPUT OPERATION Figure 2
OPERATION-Programming
There are two areas of interest in programming the DS1721: the Configuration/Status register and the
thermostat setpoints. All programming is done via the 2-wire interface using the protocols discussed in
the “Command Set” section.
Configuration/Status Register Programming
The configuration/status register is accessed via the Access Config (ACh) function command. Writing to
or reading from the register is determined by the R/ W bit of the 2-wire control byte (See “2-wire Serial
Data Bus” section). Data is read from or written to the configuration register MSb first. The format of the
register is illustrated below in Figure 3. The effect each bit has on DS1721 functionality is described
below along with the power-up state of the bit and its ability to be read or written to. The entire register is
volatile and will always power-up in the default state. Therefore, it is recommended that the user issue
any configuration programming commands immediately after power is cycled, before any other
commands are issued.
CONFIGURATION/STATUS REGISTER Figure 3
DONE X X U R1 R0 POL 1SHOT
MSb LSb
1SHOT = Temperature Conversion Mode. If 1SHOT is "1", the DS1721 will perform and store one
temperature conversion upon reception of the Start Convert T (51h) command. If 1SHOT is "0", the
DS1721 will continuously perform temperature conversions and store the last completed result in the
Thermometer Register. The user has read/write access to the bit and the power-up default state is "0"
(continuous mode).
POL = TCOM Polarity Bit. If POL is "1", the active state of the TCOM output will be high. A "0" stored
in this location sets the thermostat output to an active low state. The user has read/write access to the
POL bit, and the power-up default state is "1" (active high).
U = Undefined. This bit is used internally by the DS1721. It will be a "0" at power-up and will change to
a "1" once the Start Convert T [51h] command is issued. This is a “Don’t Care” on a write; i.e. The
DS1721 will ignore writes to this location.
DS1721
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R0, R1 = Thermometer Resolution Bits. Table 3 below defines the resolution of the digital thermometer,
based on the settings of these two bits. There is a direct tradeoff between resolution and conversion time,
as depicted in the DC Electrical Characteristics: Digital Thermometer table. The designer has read/write
access to R0 and R1, and the default state is R0="1" and R1="1" (12-bit conversions).
THERMOMETER RESOLUTION CONFIGURATION Table 3
R1
R0
THERMOMETER
RESOLUTION
MAX
CONVERSION
TIME
0 0 9-BIT 93.75ms
0 1 10-BIT 187.5ms
1 0 11-BIT 375ms
1 1 12-BIT 750ms
X = Undefined. These bits are used internally by the DS1721.
DONE = Temperature Conversion Status Bit. "1" = conversion complete and "0" = conversion in
progress. The DONE bit is read-only, and the power-up state is "1". In the continuous conversion mode,
DONE = "0".
Thermostat Setpoints Programming
The thermostat registers (TH and TL) define the setpoints for operation of the TCOM output. The
respective register can be accessed over the 2-wire bus via the Access TH (A1h) or Access TL (A2h)
commands. Reading from or writing to the respective register is controlled by the state of the R/ W bit in
the 2-wire control byte (See “2-Wire Serial Data Bus” section).
The format of the TH and TL registers is a 12-bit 2’s complement representation of the temperature in °C.
The user can program the number of bits (9, 10, 11, or 12) for each TH and TL that correspond to the
thermometer resolution configuration chosen. If the 9-bit mode is chosen, for example, the 3 least
significant bits of TH and TL will be ignored by the thermostat comparator. The format for both TH and
TL is shown in Figure 4. The power-up default of TH is 80°C and that for TL is 75°C.
TEMPERATURE/DATA RELATIONSHIPS Figure 4
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb (UNIT = °C) LSb
2
-1
2
-2
2
-3
2
-4
0 0 0 0 LSB
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+80°C 0101 0000 0000 0000 5000h
+75°C 0100 1011 0000 0000 4B00h
+10.125°C 0000 1010 0010 0000 0A20h
+0.5°C 0000 0000 1000 0000 0080h
+0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1000 0000 FF80h
-10.125°C 1111 0101 1110 0000 F5E0h
-25.0625°C 1110 0110 1111 0000 E6F0h
-55°C 1100 1001 0000 0000 C900h
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DS1721S

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