DS1721 Datasheet, PDF (2/16 Pages) Dallas Semiconductor – 2-Wire Digital Thermometer and Thermostat

DS1721
DETAILED PIN DESCRIPTION Table 1
PIN SYMBOL DESCRIPTION
1
SDA Data input/output pin. For 2-wire serial communication port.
2
SCL Clock input/output pin. For 2-wire serial communication port.
3
T_OUT Thermostat output. Active when temperature exceeds TH; will reset when
temperature falls below TL.
4
GND Ground pin.
5
A2
Address input pin.
6
A1
Address input pin.
7
A0
Address input pin.
8
V_DD Supply Voltage. 2.7V to 5.5V input power pin.
OVERVIEW
A block diagram of the DS1721 is shown in Figure 1.
The DS1721 consists of five major components:
1. Precision temperature sensor
2. Analog-to-digital converter
3. 2-wire interface electronics
4. Data registers
5. Thermostat comparator
The factory-calibrated temperature sensor requires no external components. Upon power-up, the DS1721
is in an idle mode. Upon issuance of a Start Convert T command [51h], the DS1721 begins temperature
conversions with the default resolution of 12 bits (0.0625°C resolution). Following an 8-bit command
protocol, temperature data can be read over the 2-wire interface. The host can periodically read the value
in the temperature register, which contains the last completed conversion. As conversions are performed
in the background, reading the temperature register does not affect the conversion in progress.
In power-sensitive applications, the user can put the DS1721 in a “one-shot” mode, under which the
sensor will complete and store one temperature conversion and return to a low-power standby state. In
time-sensitive applications, the user can change the conversion resolution from 12 bits to 9, 10, or 11.
Each additional bit of resolution approximately doubles the conversion time, so 9-bit conversions can be
performed in less than a quarter of a second.
This is accomplished by issuing a command protocol to the configuration register. It is recommended that
the user issue the command protocol to program the configuration register before any temperature
conversion commands are issued after power-up. This is due to the fact that the configuration data is
stored in volatile memory and will always power-up in the default state. The configuration register
defines the conversion mode, thermometer resolution/conversion time, and active state of the thermostat
comparator output. It also contains 3 status bits denoting the state of temperature conversions and
thermostat flags.
The user can also program over-temperature (TH) and under-temperature (TL) setpoints for thermostatic
operation. The power-up state of TH is 80°C and that for TL is 75°C. The result of each temperature
conversion is compared with these setpoints. The thermostat output (T_OUT) becomes active when the
measured temperature exceeds the programmed TH, and remains latched in the active state until
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